The third pathway: endothelium-dependent hyperpolarization.

In response to various neurohumoral substances endothelial cells release nitric oxide (NO), prostacyclin and produce hyperpolarization of the underlying vascular smooth muscle cells, possibly by releasing another factor termed endothelium-derived hyperpolarizing factor (EDHF). EDHF-mediated responses are sensitive to the combination of two toxins, charybdotoxin plus apamin, but do not involve ATP-sensitive or large conductance calcium-activated potassium channels. As hyperpolarization of the endothelial cells is required in order to observe endothelium-dependent hyperpolarization, and electrical coupling through myo-endothelial gap junctions may explain the phenomenon. An alternative explanation is that the hyperpolarization of the endothelial cells causes an efflux of potassium that in turn activates the inwardly rectifying potassium conductance and the Na+/K+ pump of the smooth muscle cells. Endothelial cells produce metabolites of the cytochrome P450-monooxygenase that activate BKCa, and induce hyperpolarization of coronary arterial smooth muscle cells. The elucidation of the mechanism underlying endothelium-dependent hyperpolarization and the discovery of specific inhibitors of the phenomenon are prerequisite for the understanding of the physiological role of this alternative endothelial pathway involved in the control of vascular tone in health and disease.     

Communication between endothelial and smooth muscle cells

Vascular endothelial cells play a fundamental role in the control of vascular tone, and therefore in the control of local blood flow, by releasing various contracting (endothelin, prostaglandins) and relaxing (prostacycline, NO) factors. An additional mechanism involving the hyperpolarization of the vascular smooth muscle cells is observed mainly in the coronary vascular bed and in the periphery. This phenomenon was attributed to an elusive endothelial factor called endothelium-derived hyperpolarizing factor (EDHF). This mechanism is now better understood. It involves first an increase in the endothelial intracellular concentration of calcium, the activation of endothelial potassium channels and the resulting hyperpolarization of the endothelial cells. The hyperpolarization of the endothelial cells is transmitted to the smooth muscle cells by different pathways. This hyperpolarization propagates along the vessels not only via the smooth muscle cells but also via the endothelial cells. Therefore, the endothelial layer can also be considered as a conducting tissue. The discovery of specific inhibitors of the endothelial cell hyperpolarization allows the assessment of the contribution of EDHF-mediated responses in the control of vascular tone.     

Potassium effects on contraction in arterial smooth muscle mediated by Na+, K+-ATPase

A local increase in the extracellular potassium concentration [K+]o, up to about 8 meq/liter either by topical application or intra-arterial infusion of K+ salts, causes arteriolar dilation and decreased resistance to blood flow in systemic vascular beds. Isolated vascular smooth muscle responds to a similar increase in [K+] in the bathing fluid with relaxation if the preparation has some initial active tension. Reduction in [K+] over physiological ranges produces arteriolar constriction and increased resistance to blood flow. K+ vasodilation is accompanied by hyperpolarization of the smooth muscle cell whereas the vasoconstriction is accompanied by depolarization. All these responses can be blocked by ouabain, a potent Na+, K+-ATPase inhibitor. It is therefore thought that K+ vasodilation results from stimulation of the electrogenic Na+-K+ pump whereas the vasoconstriction results from inhibition of this pump. A number of conditions that alter resistance also alter interstitial fluid [K+]. These include exercise, myocardial ischemia, epileptic convulsions, and evoked electrical activity of the somatomotor cortex. Certain findings, including those during administration of ouabain, suggest that changes in [K+] contribute significantly to some of the changes in resistance.     

Origins of blood volume change due to glutamatergic synaptic activity at astrocytes abutting on arteriolar smooth muscle cells

The cellular mechanisms that couple activity of glutamatergic synapses with changes in blood flow, measured by a variety of techniques including the BOLD signal, have not previously been modelled. Here we provide such a model, that successfully accounts for the main observed changes in blood flow in both visual cortex and somatosensory cortex following their stimulation by high-contrast drifting grating or by single whisker stimulation, respectively. Coupling from glutamatergic synapses to smooth muscle cells of arterioles is effected by astrocytes releasing epoxyeicosatrienoic acids (EETs) onto them, following glutamate stimulation of the astrocyte. Coupling of EETs to the smooth muscle of arterioles is by means of potassium channels in their membranes, leading to hyperpolarization, relaxation and hence an increase in blood flow. This model predicts a linear increase in blood flow with increasing numbers of activated astrocytes, but a non-linear increase with increasing glutamate release.     

Neutrophil hyperpolarization in response to a chemotactic peptide

The chemotactic peptide formylmethionyl-leucyl-phenylalanine (fMLP), at concentrations below 10(-9) M, elicits a sustained increase in the human neutrophil's membrane potential within 10 s of its addition. This hyperpolarization, detected with the fluorescent cationic potentiometric probes, 3,3'-dipentyloxacarbocyanine (diO-C5-(3)), and 1,1'-dipropyl-3,3,3',3'-tetramethylindocarbocyanine iodide (diI-C3-(3)), and with the anionic probe bis-(1,3-diethylthiobarbituric)trimethine oxonol (bis-oxonol), is immediately followed by a large depolarization when [fMLP] greater than 10(-9) M. By extracellular substitution of sodium ions with potassium ions or choline or by pretreatment of the cells with ionophores, we report here that the hyperpolarization is primarily dependent on an intact potassium ion gradient and is accompanied by a concurrent acidification of the cytoplasm (approximately 0.05 pH unit) Although the latter occurs simultaneously with a large, transient increase in cytosolic Ca2+ at [fMLP] greater than 10(-10) M, it occurs without a detectable increase in cytosolic Ca2+ at [fMLP] less than 10(-10) M. The hyperpolarization is neither affected nor initiated by the chemotactic peptide antagonist tert-butyloxycarbonyl-methionyl-leucyl-phenylalanine, whereas the depolarization is completely inhibited. Neutrophils isolated from patients with X-linked chronic granulomatous disease exhibit normal hyperpolarizations and cytosolic Ca2+ increases in response to chemotactic peptides but exhibit no depolarization or oxidative burst. The hyperpolarization appears earlier in the ontogeny of differentiating myeloid precursor cells than either the rise in cytosolic Ca2+ or the depolarization response. Together, these findings indicate that an increase in transmembrane potential is one of the earliest events in the neutrophil response to chemotactic peptides, coinciding temporally with increases in cytoplasmic Ca2+ and H+ concentrations but preceding detectable oxidative burst activity.     

Potassium (BK(Ca)) currents are reduced in microvascular smooth muscle cells from insulin-resistant rats

Insulin resistance (IR) syndrome is associated with impaired vascular relaxation; however, the underlying pathophysiology is unknown. Potassium channel activation causes vascular smooth muscle hyperpolarization and relaxation. The present study determined whether a reduction in large conductance calcium- and voltage-activated potassium (BK(Ca)) channel activity contributes to impaired vascular relaxation in IR rats. BK(Ca) channels were characterized in mesenteric microvessels from IR and control rats. Macroscopic current density was reduced in myocytes from IR animals compared with controls. In addition, inhibition of BK(Ca) channels with tetraethylammonium (1 mM) or iberiotoxin (100 nM) was greater in myocytes from control (70%) compared with IR animals (approximately 20%). Furthermore, activation of BK(Ca) channels with NS-1619 was three times more effective at increasing outward current in cells from control versus IR animals. Single channel and Western blot analysis of BK(Ca) channels revealed similar conductance, amplitude, voltage sensitivity, Ca2+ sensitivity, and expression density between the two groups. These data provide the first direct evidence that microvascular potassium currents are reduced in IR and suggest a molecular mechanism that could account for impaired vascular relaxation in IR.     

Time course of vasodilatory responses in skeletal muscle arterioles: role in hyperemia at onset of exercise

At the onset of dynamic exercise, muscle blood flow increases within 1-2 s. It has been postulated that local vasodilatory agents produced by the vascular endothelium or the muscle itself contribute to this response. We hypothesized that only vasodilators that act directly on the vascular smooth muscle could produce vasodilation of skeletal muscle arterioles in <2 s. To test this hypothesis, we determined the time course of the vasodilatory response of isolated skeletal muscle arterioles to direct application of potassium chloride, adenosine, acetylcholine, and sodium nitroprusside. Soleus and gastrocnemius muscles were dissected from the hindlimbs of male Sprague-Dawley rats. First-order arterioles (100-200 microm) were isolated, cannulated on micropipettes, and pressurized to 60 cmH(2)O in an organ bath. Vasodilatory agents were added directly to the bath, and diameter responses of the arterioles were recorded in real time on a videotape recorder. Frame-by-frame analysis of the diameter responses indicated that none of the vasodilator agents tested produced significant diameter increases in <4 s in either soleus or gastrocnemius muscle arterioles. These results indicate that, although these local vasodilators produce significant vasodilation of skeletal muscle resistance arterioles, these responses are not rapid enough (within 1-2 s) to contribute to the initiation of the exercise hyperemic response at the onset of dynamic exercise.     

Hemorheologic events in severe shock

Persistent low perfusion and low blood pressure are the two major events in the pathogenesis of irreversible shock. This review is focused on our recent study on the mechanism of, and a new therapeutic approach to the two events in IS. One of the main causes of persistent low perfusion are leukocyte adhesion on venule walls and plugging in capillaries which comes from the low wall shear stress or shear rate, and high leukocyte-endothelial adhesion force in IS. However, blockade of leukocyte adhesion by monoclonal antibodies against the adhesion molecules can only attenuate the number of sticking WBC in venules, but cannot make an appreciable improvement in capillary reflow and survival rate in IS, because it is difficult for the agents to flow into an obstructed capillary. We have shown that the administration of Polydatin, a crystalline product isolated from a traditional Chinese medicine, can restore the pulse pressure with high survival rate in irreversible shock. With an increase in pulse pressure, and the highly dispersive force resulting from pulsatile blood flow, the stationary blood cells can be pushed away from the obstructed capillary and thus promote capillary reflow. Therefore, enhancement of pulse pressure is a key factor for the treatment of low perfusion in irreversible shock. Hyperpolarization of arteriolar smooth muscle cells occurs in irreversible shock, which inhibits the potential-operated calcium channel and the influx of Ca2+ in arteriolar smooth muscle cells stimulated by norepinephrine, and finally leads to low vascular contractile responsiveness with refractory hypotension in irreversible shock. Activation of the potassium channels K(ATP) and BK(Ca) is involved in arteriolar smooth muscle cells hyperpolarization. In irreversible shock, ATP depletion, intracellular acidosis, ONOO- formation, and enhancement of a calcium spark results in activation of K(ATP) and BK(Ca) and consequent arteriolar smooth muscle cell hyperpolarization. Therefore, a new therapeutic strategy for refractory hypotension was suggested, including blockade of potassium channel activation to reconstitute vasoreactivity and the administration of vasopressors to elevate blood pressure in the treatment of irreversible shock.     

High-calcium diet enhances vasorelaxation in nitric oxide-deficient hypertension

Because the effects of calcium supplementation on arterial tone in nitric oxide-deficient hypertension are unknown, we investigated the influence of elevating dietary calcium from 1.1 to 3.0% in Wistar rats treated with N(G)-nitro-L-arginine methyl ester (L-NAME; 20 mg. kg(-1). day(-1)) for 8 wk. A high-calcium diet attenuated the development of hypertension induced by L-NAME and abrogated the associated impairments of endothelium-independent mesenteric arterial relaxations to nitroprusside, isoproterenol, and cromakalim. Endothelium-dependent relaxations to acetylcholine during nitric oxide synthase inhibition in vitro were decreased in L-NAME rats and improved by calcium supplementation. The inhibition of cyclooxygenase by diclofenac augmented the responses to acetylcholine in L-NAME rats but not in calcium + L-NAME rats. When hyperpolarization of smooth muscle was prevented by KCl precontraction, the responses to acetylcholine during combined nitric oxide synthase and cyclooxygenase inhibition were similar in all groups. Furthermore, superoxide dismutase enhanced the acetylcholine-induced relaxations in L-NAME rats but not in calcium + L-NAME rats. In conclusion, calcium supplementation reduced blood pressure during chronic nitric oxide synthase inhibition and abrogated the associated impairments in endothelium-dependent and -independent arterial relaxation. The augmented vasorelaxation after increased calcium intake in L-NAME hypertension may be explained by enhanced hyperpolarization and increased sensitivity to nitric oxide in arterial smooth muscle and decreased vascular production of superoxide and vasoconstrictor prostanoids.     

Acute systemic hypoxia elevates venous but not interstitial potassium of dog skeletal muscle

Potassium release through ATP-sensitive potassium (K(ATP)) channels contributes to hypoxic vasodilation in the skeletal muscle vascular bed: It is uncertain whether K(ATP) channels on muscle cells contribute to the process. Potassium from muscle cells must cross the interstitial space to reach the vascular tissues, whereas that from vascular endothelium would have a higher concentration in venous blood than in interstitial fluid. We determined the effect of systemic hypoxia on arterial, venous, and interstitial potassium in the constant-flow-perfused gracilis muscles of anesthetized dogs. Hypoxia reduced arterial Po(2) from 138 to 25 and Pco(2) from 28 to 26 mmHg. Arterial pH and potassium were well correlated (r(2) = 0.9): Both increased in early hypoxia and decreased during the postcontrol. In denervated muscles, perfusion pressure decreased from 95 to 76 mmHg by the end of the hypoxic period; neither venous nor interstitial potassium was elevated. In innervated muscles, perfusion pressure increased from 110 to 172 mmHg by the 11th min of hypoxia and then decreased to 146 mmHg by the end of the hypoxic period; venous potassium increased from 5.0 to 5.3 mM, but interstitial potassium remained unchanged. Glibenclamide abolished both the increase in venous potassium and the hypoxic vasodilation in the innervated muscle. Thus skeletal muscle cells were unlikely to have contributed to the release of potassium, which was suggested to originate from vascular endothelium. The sympathetic nerve supply may play a direct or indirect role in the opening of K(ATP) channels under hypoxic conditions.     

Potassium depletion selectively inhibits sustained diacylglycerol formation from phosphatidylinositol in angiotensin II-stimulated, cultured vascular smooth muscle cells

Potassium depletion decreases blood pressure in vivo and blunts the pressor response to angiotensin II (ang II) without down-regulating the receptor. In cultured rat aortic smooth muscle cells, the ang II-induced signaling sequence is biphasic with rapid hydrolysis of the polyphosphoinositides producing an early (15 s) diacylglycerol (DG) peak and a transient rise in inositol trisphosphate (IP3) and more delayed phosphatidylinositol (PI) hydrolysis resulting in sustained DG formation (peak at 5 min). Exposure of intact vascular smooth muscle cells to low potassium growth medium for 24 h or acutely potassium-depleting cells with nigericin causes selective, marked inhibition of late DG formation (5-min peak inhibited by 60 +/- 8% and 84 +/- 7%, respectively). The early cell response, namely polyphosphoinositide hydrolysis, inositol bis- and trisphosphate production and the 15-s DG peak, is not affected. Analysis of 125I-ang II-binding data reveals no significant differences in either receptor number or binding affinity (Kd) in potassium-depleted cells. Together with its marked inhibitory effect on sustained ang II-induced DG formation, acute potassium depletion effectively blocks internalization of 125I-ang II: there is no significant internalization of the ligand after 5 min at 37 degrees C versus 64 +/- 7% internalization in control cells. Thus, potassium depletion does not alter ang II binding or initial membrane signaling in rat aortic smooth muscle but blocks ligand internalization and selectively and markedly inhibits the development of direct PI hydrolysis and sustained diacylglycerol formation. These findings suggest a role for ligand-receptor processing in generating the sustained cell response and potentially explain the lower blood pressure and decreased pressor response to ang II seen in hypokalemic states in vivo. Furthermore, the ability of K+ depletion to alter secondary signal generation may provide insight into the mechanisms underlying the K+ dependence of a variety of cell functions.     

Changes in the content of potassium, sodium and potassium influx in human peripheral blood lymphocytes during long-term cultivation with phytohemagglutinin

Intracellular potassium, sodium and potassium influx were examined in PHA-activated human lymphocytes within 6 days of cultivation. DNA flow cytometry was used to estimate the percentage of cells in G1, S and G2 + M phases. Potassium influx and content per g protein were found to be increased, whereas sodium content decreased with the progression of cells from G1 to S phases, being maximum on the 3rd day. Later on the percentage of cells in S phase was seen diminished, and the potassium content decreased just as sodium content increased. It is concluded that ionic changes may correlate with the entering of cells into S phase.     

Potassium Buffering in the Neurovascular Unit: Models and Sensitivity Analysis

Astrocytes are critical regulators of neural and neurovascular network communication. Potassium transport is a central mechanism behind their many functions. Astrocytes encircle synapses with their distal processes, which express two potassium pumps (Na-K and NKCC) and an inward rectifying potassium channel (Kir), whereas the vessel-adjacent endfeet express Kir and BK potassium channels. We provide a detailed model of potassium flow throughout the neurovascular unit (synaptic region, astrocytes, and arteriole) for the cortex of the young brain. Our model reproduces several phenomena observed experimentally: functional hyperemia, in which neural activity triggers astrocytic potassium release at the perivascular endfoot, inducing arteriole dilation; K⁺ undershoot in the synaptic space after periods of neural activity; neurally induced astrocyte hyperpolarization during Kir blockade. Our results suggest that the dynamics of the vascular response during functional hyperemia are governed by astrocytic Kir for the fast onset and astrocytic BK for maintaining dilation. The model supports the hypothesis that K⁺ undershoot is caused by excessive astrocytic uptake through Na-K and NKCC pumps, whereas the effect is balanced by Kir. We address parametric uncertainty using high-dimensional stochastic sensitivity analysis and identify possible model limitations.     

Potassium Buffering in the Neurovascular Unit: Models and Sensitivity Analysis

Astrocytes are critical regulators of neural and neurovascular network communication. Potassium transport is a central mechanism behind their many functions. Astrocytes encircle synapses with their distal processes, which express two potassium pumps (Na-K and NKCC) and an inward rectifying potassium channel (Kir), whereas the vessel-adjacent endfeet express Kir and BK potassium channels. We provide a detailed model of potassium flow throughout the neurovascular unit (synaptic region, astrocytes, and arteriole) for the cortex of the young brain. Our model reproduces several phenomena observed experimentally: functional hyperemia, in which neural activity triggers astrocytic potassium release at the perivascular endfoot, inducing arteriole dilation; K⁺ undershoot in the synaptic space after periods of neural activity; neurally induced astrocyte hyperpolarization during Kir blockade. Our results suggest that the dynamics of the vascular response during functional hyperemia are governed by astrocytic Kir for the fast onset and astrocytic BK for maintaining dilation. The model supports the hypothesis that K⁺ undershoot is caused by excessive astrocytic uptake through Na-K and NKCC pumps, whereas the effect is balanced by Kir. We address parametric uncertainty using high-dimensional stochastic sensitivity analysis and identify possible model limitations.     

Muscle pump does not enhance blood flow in exercising skeletal muscle.

The muscle pump theory holds that contraction aids muscle perfusion by emptying the venous circulation, which lowers venous pressure during relaxation and increases the pressure gradient across the muscle. We reasoned that the influence of a reduction in venous pressure could be determined after maximal pharmacological vasodilation, in which the changes in vascular tone would be minimized. Mongrel dogs (n = 7), instrumented for measurement of hindlimb blood flow, ran on a treadmill during continuous intra-arterial infusion of saline or adenosine (15-35 mg/min). Adenosine infusion was initiated at rest to achieve the highest blood flow possible. Peak hindlimb blood flow during exercise increased from baseline by 438 +/- 34 ml/min under saline conditions but decreased by 27 +/- 18 ml/min during adenosine infusion. The absence of an increase in blood flow in the vasodilated limb indicates that any change in venous pressure elicited by the muscle pump was not adequate to elevate hindlimb blood flow. The implication of this finding is that the hyperemic response to exercise is primarily attributable to vasodilation in the skeletal muscle vasculature.     

The effect of intravenous infusion of hyperosmotic sodium and potassium chloride solutions on cephalic blood flow in conscious sheep.

The rate of flow of plasma and blood through the head of conscious sheep was measured before, during and after the intravenous infusion of 1 mol. 1(-1) NaCl and 1 mol. 1(-1) KCl at 0-8--1-0 ml. min-1 for 2 hours. The plasma flow was estimated by indicator-dilution technique using sodium para-aminohippurate which was shown to be a satisfactory indicator substance. Short periods of rumination were found to cause marked increases in cephalic blood flow. The infusion of hyperosmotic sodium chloride caused no consistent changes in the rates of cephalic plasma flow and blood flow. During potassium infusion plasma and blood flows increased as the plasma potassium concentration increased up to approximately 6 mmol.1(-1). Further increases in plasma potassium concentration were associated with a progressive return of these flow rates to or below the pre-infusion levels. This pattern of change in the rate of plasma flow through the head of the sheep was very similar to that previously reported for renal plasma flow during hyperkalaemia in conscious sheep. At its maximum the cephalic plasma flow was 1-163+/-0-029 (S.E. of mean) times the pre-infusion flow rate. Cephalic blood flow tended to reach maximum rates at slightly higher plasma potassium concentrations and thereafter, to fall more slowly than the plasma flow due to concomitant increases in haematocrit. Maximum cephalic blood flow was 1-176+/-0-032 times the pre-infusion flow rate. The lowest rates of cephalic plasma and blood flow occurred during the first 30 minutes following cessation of potassium infusion.     

Role of Ba2+-resistant K+ channels in endothelium-dependent hyperpolarization of rat small mesenteric arteries

In rat small mesenteric arteries, the influence of modulation of basal smooth muscle K+ efflux on the mechanism of endothelium-dependent hyperpolarization was investigated. The membrane potentials of the vascular smooth muscle cells were measured using conventional microelectrode techniques. Incubation of resting arteries with the gap junction uncoupler carbenoxolone (20 micro M) decreased the endothelium-dependent hyperpolarization elicited by a submaximal concentration of acetylcholine (3 micro M) to about 65% of the control. In the presence of Ba2+ (200 micro M), which depolarized the membrane potential by 10 mV, the acetylcholine-induced membrane potential response was doubled in magnitude, reaching values not different from control. Moreover, the hyperpolarization was more resistant to carbenoxolone in these conditions. Finally, both in the absence and in the presence of carbenoxolone, the combined application of Ba2+ and ouabain (0.5 mM) did not abolish the acetylcholine response. These results suggest that gap junctional coupling plays a role in endothelium-dependent hyperpolarization of smooth muscle cells of resting rat small mesenteric arteries. Additionally, these findings show that the hyperpolarization does not rely on activation of inward rectifying K+ channels. Although a minor contribution of Na-K pumping cannot be excluded, the Ba2+ experiments show that the membrane electrical response is mediated by activation of a Ba2+-resistant K+ conductance.     

Cyclosporine augments reactivity of isolated blood vessels

Administration of cyclosporine (CS) as an immunosuppressive agent in clinical transplantation is associated with multiple side effects including nephrotoxicity and hypertension. These two effects could be related in that the renal changes may be secondary to alterations in organ blood flow. The present studies investigate the ability of CS to augment contractile responsiveness in blood vessels from normotensive rats. Isometric force generation was measured in isolated tail arteries and portal veins. CS (8.3×10⁻⁶M) potentiated tail artery contractile responses to sympathetic nerve stimulation, exogenous norepinephrine, and increases in extracellular potassium concentration. Portal veins undergo spontaneous contractions which are related to the firing of calcium-driven action potentials in the smooth muscle cells. CS significantly increased the frequency of these spontaneous contractile events. These results suggest that components of CS toxicity may involve a direct action on vascular smooth muscle and/or on vascular adrenergic neurotransmission.     

Inhibitory and excitatory mechanisms of neurotensin action in canine intestinal circular muscle in vitro.

The effect of neurotensin on canine ileal circular muscle devoid of myenteric plexus was investigated using single and double sucrose gap techniques. Similar results were obtained with microelectrode techniques. Neurotensin caused a temperature-sensitive and dose-dependent biphasic response, an initial hyperpolarization associated with inhibition of contractile activity, followed by an excitatory phase, usually consisting of spike discharge and tonic and phasic contractions, for which depolarization was not required. Neither response was affected by tetrodotoxin, phentolamine, propranolol, or atropine. The hyperpolarization was associated with decreased membrane resistance, blocked by 10(-7) M apamin, and converted to tonic depolarization by apamin (10(-6) M). Tachyphylaxis to neurotensin occurred when the stimulation interval was less than 20 min. After Ca2+ depletion, depolarization was observed instead of the hyperpolarization; this depolarization was not affected by nitrendipine and was gradually abolished with repetitive stimulation at 20-min intervals. When Ca2+ was present, nifedipine did not alter the hyperpolarizing phase of the response but inhibited spiking and blocked all contractions. The excitatory phase of the response was enhanced by Bay K-8644. Neuromedin N elicited a response identical with that of neurotensin. The responses of the two peptides were completely cross tachyphylactic. Inhibitory junction potentials were not affected by neurotensin tachyphylaxis. It is concluded that neurotensin and neuromedin N activate apamin-sensitive, calcium-dependent potassium channels in circular muscle, causing membrane hyperpolarization and inhibition of muscle contraction. Release of intracellular calcium is involved in the activation of these potassium channels.(ABSTRACT TRUNCATED AT 250 WORDS)     

Unusual potassium channels mediate nonadrenergic noncholinergic nerve-mediated inhibition in opossum esophagus

Field stimulation of the circular muscle of the opossum esophagus produces a transient hyperpolarization (inhibitory junction potential, IJP) followed by an "off" depolarization. A similar nonadrenergic, noncholinergic (NANC) response in guinea pig taenia caecum has been shown to be due to an increase in the potassium ion permeability of the smooth muscle cell membrane. Double sucrose gap studies showed a decrease in resistance during the IJP, and a reversal at an estimated membrane potential of about -90 mV (4 mM K+). The reversal potential was dependent on the extracellular potassium concentration, shifting to -75 mV when the potassium in the superfusion medium was increased to 10 mM. The IJP in the opossum esophageal circular smooth muscle is therefore like the IJP of the guinea pig taenia caecum in that it is probably due to a selective increase in potassium ion permeability. Potassium conductance blocking agents, tetraethylammonium chloride (TEA, 20 mM) and 4-aminopyridine (4-AP, 5 mM) both caused a depolarization of the smooth muscle cell membrane, but TEA increased the membrane resistance, whereas 4-AP did not affect the membrane conductance in a consistent way. A decrease in IJP amplitude owing to these agents was not apparent. Apamin (10 microM) did not affect the membrane potential, the membrane resistance, or the IJP. Quinine (0.1 mM) produced effects quantitatively similar to those of TEA. Quinine (1 mM) did abolish the IJP, however, this was likely due to a blockade of impulse transmission of the intramural nerves.(ABSTRACT TRUNCATED AT 250 WORDS)