Nicardipine as a Ca2+ channel blocker in single barnacle muscle fibers

A study has been made of the efficacy of nicardipine as a Ca2+ channel blocker by determining the magnitude of its effect on the stimulatory response of the ouabain-insensitive Na+ efflux in single barnacle muscle fibers to 100 mM external K+. The results show that nicardipine (at pH 6.5) is a potent inhibitor, the minimal effective concentration being approx. 10(-7) M and the IC(50) about 5.10(-6) M. Nicardipine, however, is not as potent as verapamil (at pH 6.5) on an equimolar basis. This is explained by assuming that the number of dihydropyridine receptors in the t-tubule membranes of barnacle fibers is not high or that verapamil is able to block the sarcoplasmic reticulum Ca2+ release channel in addition to the voltage-dependent Ca2+ channels.     

Pertussis toxin pretreatment abolishes dihydropyridine inhibition of calcium flux in the 235-1 pituitary cell line

In the present study we used 235-1 cells, a prolactin secreting clone derived from a pituitary tumor. In these cells maitotoxin, a calcium channels activator, likely acting on voltage sensitive calcium channels, increases intracellular free calcium measured by Quin 2 technique. Maitotoxin stimulation of calcium flux was inhibited both by nicardipine and verapamil in a dose dependent manner. Pertussis toxin pretreatment does not modify maitotoxin activation of calcium channels, while completely abolishes nicardipine inhibition of maitotoxin induced voltage sensitive calcium channels activation, without affecting verapamil effect. These results suggest a possible involvement of a pertussis toxin sensitive G protein in dihydropyridine inhibition of voltage sensitive calcium channels.     

Attenuated arterial baroreflex buffering of muscle metaboreflex in heart failure

Previous studies have shown that heart failure (HF) or sinoaortic denervation (SAD) alters the strength and mechanisms of the muscle metaboreflex during dynamic exercise. However, it is still unknown to what extent SAD may modify the muscle metaboreflex in HF. Therefore, we quantified the contribution of cardiac output (CO) and peripheral vasoconstriction to metaboreflex-mediated increases in mean arterial blood pressure (MAP) in conscious, chronically instrumented dogs before and after induction of HF in both barointact and SAD conditions during mild and moderate exercise. The muscle metaboreflex was activated via partial reductions in hindlimb blood flow. After SAD, the metaboreflex pressor responses were significantly higher with respect to the barointact condition despite lower CO responses. The pressor response was significantly lower in HF after SAD but still higher than that of HF in the barointact condition. During control experiments in the barointact condition, total vascular conductance summed from all beds except the hindlimbs did not change with muscle metaboreflex activation, whereas in the SAD condition both before and after induction of HF significant vasoconstriction occurred. We conclude that SAD substantially increased the contribution of peripheral vasoconstriction to metaboreflex-induced increases in MAP, whereas in HF SAD did not markedly alter the patterns of the reflex responses, likely reflecting that in HF the ability of the arterial baroreflex to buffer metaboreflex responses is impaired.     

Comparative effects of cardiac receptors and sinoaortic baroreceptors on elevations of plasma vasopressin and renin activity elicited by haemorrhage

Increases in plasma vasopressin and renin activity that occur in response to haemorrhage have been attributed in part to reflex effects from cardiac receptors and sinoaortic baroreceptors, but the relative importance of these different receptors in causing humoral changes during haemorrhage in conscious dogs has not been reported. We investigated this question by hemorrhaging 6 sham-operated (SO), 6 cardiac-denervated (CD), 4 sinoaortic-denervated (SAD), and 4 combined sinoaortic and cardiac-denervated (SACD), conscious dogs. Blood was removed at a rate of 0.9 ml/kg X min. Plasma vasopressin and renin samples were taken during a control period and after 10, 20, and 30 ml/kg of blood had been removed. Results (mean +/- SE) are shown in the tables below. (table; see text) These experiments illustrate that: resting plasma levels of vasopressin and renin in conscious dogs are unaffected by the denervation procedures used in these experiments, the increase in plasma vasopressin that occurs during haemorrhage is mediated largely via cardiac receptors, with a considerably smaller contribution from the sinoaortic baroreceptors, during moderately severe haemorrhage (30 ml/kg) vasopressin secretion can be increased by a mechanism independent of sinoaortic and cardiac reflexes, the increase in plasma renin activity that occurs during haemorrhage is not dependent upon either cardiac or sinoaortic reflexes.     

Arterial baroreceptors control plasma vasopressin responses to graded hypotension in conscious dogs

We studied the role of cardiac and arterial baroreceptors in the reflex control of arginine vasopressin (AVP) and renin secretion during graded hypotension in conscious dogs. The dogs were prepared with Silastic cuffs on the thoracic inferior vena cava and catheters in the pericardial space. Each experiment consisted of a control period followed by four periods of inferior vena caval constriction, during which mean arterial pressure (MAP) was reduced in increments of approximately 10 mmHg. The hormonal responses were measured in five dogs under four treatment conditions: 1) intact, 2) acute cardiac denervation (CD) by intrapericardial infusion of procaine, 3) after sinoaortic denervation (SAD), and 4) during combined SAD+CD. The individual slopes relating MAP to plasma AVP and plasma renin activity (PRA) were used to compare the treatment effects using a 2 x 2 factorial analysis. There was a significant (P < 0.01) effect of SAD on the slope relating plasma AVP to MAP but no effect of CD and no SAD x CD interaction. In contrast, the slope relating PRA and MAP was increased (P < 0.05) by SAD but was not affected by CD. These results support the hypothesis that stimulation of AVP secretion in response to graded hypotension is primarily driven by unloading arterial baroreceptors in the dog.     

Different effects of L-, N- and T-type calcium channel blockers on striatal dopamine release measured by microdialysis in freely moving rats.

Using a microdialysis method, we have investigated effects of the voltage-dependent calcium channel blockers, verapamil, nicardipine, omega-conotoxin and flunarizine on the dopamine release and metabolism in the striatum of freely moving rat. Perfusion of verapamil (1-300 microM) and nicardipine (1-100 microM), an L-type calcium channel blocker, into the striatum through the dialysis membrane showed a dose-dependent decrease of dopamine release in the dialysate and slight increase of 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) levels. Treatment of omega-conotoxin (0.1, 1 microM), an N-type channel blocker, decreased about 50% basal dopamine release and slightly decreased DOPAC and HVA levels. Treatment with flunarizine (10 microM), an T-type channel blocker, did not affect the dopamine release and metabolism. From these data, it appears that treatments of the L- and N-type voltage-dependent calcium channel blockers in rat striatum suppress basal dopamine release, but T-type blocker does not suppress it, suggesting that L-, N- and T-type calcium channels regulate in vivo dopamine release in a different mechanism.     

Nicardipine inhibits axon conduction but causes dual changes of acetylcholine release in the mouse motor nerve

The effects of nicardipine, a dihydropyridine Ca2(+)-channel antagonist, on neuromuscular transmission and impulse-evoked release of acetylcholine were compared with those of nifedipine. In the isolated mouse phrenic nerve diaphragm, nicardipine (50 microM), but not nifedipine (100 microM), induced neuromuscular block, fade of tetanic contraction, and dropout or all-or-none block of end-plate potentials. Nicardipine had no significant effect on the resting membrane potential and the amplitude of miniature end-plate potentials but increased the frequency and caused the appearance of large size miniature potentials. The quantal contents of evoked end-plate potentials were increased. In the presence of tubocurarine, however, nicardipine depressed the amplitude of end-plate potentials. The compound nerve action potential was also decreased. It is concluded that nicardipine blocks neuromuscular transmission by acting on Na+ channels and inhibits axonal conduction. Nicardipine appeared to affect the evoked release of acetylcholine by dual mechanisms, i.e., an enhancement presumably by an agonist action on Ca2+ channels, like Bay K 8644 and nifedipine, and inhibition by an effect on Na+ channels, like verapamil and diltiazem. In contrast with its inactivity on the amplitude of miniature end-plate potentials, depolarization of the end plate in response to succinylcholine was greatly depressed. The contractile response of baby chick biventer cervicis muscle to exogenous acetylcholine was noncompetitively antagonized by nicardipine (10 microM), but was unaffected by nifedipine (30 microM). These results may implicate that nicardipine blocks the postsynaptic acetylcholine receptor channel by enhancing receptor desensitization or by a use-dependent effect.     

Cardiorespiratory effects of prolonged angiotensin II block in resting conscious dogs

Intravenous (iv) infusion of the angiotensin II (ANG II) receptor blocker saralasin in resting conscious dogs during physiological pertubations, such as hypotension and prolonged hypoxia, indicates the presence of an ANG II drive to increase respiration and decrease the arterial partial pressure of CO2 (PaCO2). In contrast, in eupneic resting dogs on a regular chow diet, iv infusion of saralasin for short periods (up to 30 min) provides no evidence of a tonic effect of circulating levels of ANG II on acid-base balance, respiration, metabolism, or circulation. However, ANG II influences physiological processes involving salt, water, and acid-base balances, which are potentially expressed beyond a 30 min time period, and could secondarily affect respiration. Therefore, we tested the hypothesis that blocking ANG II with iv saralasin would affect respiration and circulation over a 4-h period. Contrary to the hypothesis, iv infusion of saralasin in resting conscious eupneic dogs on a regular chow diet over a 4-h period had no effects on plasma strong ions, osmolality, acid-base balance, respiration, metabolism, or circulation when compared with similar control studies in the same animals. Thus, ANG II does not play a tonic modulatory role in respiratory control under "normal" physiological conditions.     

Cyclosporine A and purinergic receptors in rat kidney

Previous reports have demonstrated that Cyclosporine A (CyA) chronically administered induces an increase in adenosine plasma concentration by inhibiting adenosine uptake by red blood cells (RBC). We hypothesized that this effect may modulate, by a down regulation, the mRNA expression of adenosine receptors in rat kidney. Since high blood pressure (HBP) is a classical side effect of CyA treatment, nicardipine, a dihydropyridine calcium channel blocker, is often associated with CyA in treatment. To distinguish between the effects of CyA-induced HBP and the effects of CyA by itself, we have evaluated the effects of CyA and/or nicardipine on the mRNA expression of A1 and A2a adenosine receptors. The study was performed on five groups of rats (n= 8) receiving during 21 days either serum saline (0.5 ml i.p), CyA (12 mg/kg/day, i.p), nicardipine (1.2 mg/kg i.p) or nicardipine + CyA. The last (or fifth) group was injected with vehicle (0.5 ml i.p). Blood samples for adenosine assay were collected in the renal artery at day 21, just before the rat kidneys were removed for quantitation of adenosine A1 and A2a mRNA concentration by RT-PCR. We make two conclusions :i) Nicardipine induces a decrease in mRNA expression of A1 but not of A2a adenosine receptors. However, because nicardipine lowered both blood pressure and A1 mRNA expression, it is not possible to conclude if A1 mRNA decrease is implicated in the nicardipine effects on blood pressure.ii) CyA induces an increase in renal artery adenosine concentration and a decrease in mRNA expression of A1 and A2a adenosine receptors.     

Hemodynamic and norepinephrine responses to pacing-induced heart failure in conscious sinoaortic-denervated dogs

Brändle, Marian, Kaushik P. Patel, Wei Wang, andIrving H. Zucker. Hemodynamic and norepinephrine responses topacing-induced heart failure in conscious sinoaortic-denervated dogs.J. Appl. Physiol. 81(4):1855-1862, 1996.The present study was undertaken to determinethe effects of chronic sinoaortic (baroreceptor) denervation (SAD) on the hemodynamic and sympathetic alterations thatoccur in the pacing-induced model of congestive heart failure. Twogroups of dogs were examined: intact(n = 9) and SAD(n = 9). Both groups of dogs werestudied in the control (prepace) state and each week after theinitiation of ventricular pacing at 250 beats/min. After the pacemakerwas turned off, hemodynamic and plasma norepinephrine levels returnedtoward control levels in the prepaced state and after 1 and 2 wk ofpacing. However, by 3 wk all hemodynamic and norepinephrine levelsremained relatively constant over the 10-min observation period withthe pacemaker off. With the pacemaker off, left ventricularend-diastolic pressure went from 2.7 ± 1.4 (SE) mmHg during theprepace state to 23.2 ± 2.9 mmHg in the heart failure state inintact dogs (P < 0.01). Leftventricular end-diastolic pressure increased to 27.1 ± 2.2 mmHgfrom a control level of 4.2 ± 1.9 mmHg in SAD dogs(P < 0.0003). Mean arterial pressuresignificantly decreased in intact and SAD dogs. Resting heart rate wassignificantly higher in SAD dogs and increased to 135.8 ± 8.9 beats/min in intact dogs and 136.1 ± 6.5 beats/min in SAD dogs.There were no significant differences in the hemodynamic parametersbetween intact and SAD dogs after pacing. Plasma norepinephrine wassignificantly lower in intact than in SAD dogs before pacing (197.7 ± 21.6 vs. 320.6 ± 26.6 pg/ml;P < 0.005). In the heart failurestate, plasma norepinephrine increased significantly in both intact(598.3 ± 44.2 pg/ml) and SAD (644.0 ± 64.6 pg/ml) groups. Therewere no differences in the severity or the magnitude of the developedheart failure state in SAD vs. intact dogs. We conclude from these datathat the arterial baroreflex is not the sole mechanism for the increasein sympathetic drive in heart failure.

     

Comparison of effects of a potassium channel opener BRL34915, a specific potassium ionophore valinomycin and calcium channel blockers on endothelin-induced vascular contraction

The effects of a potassium (K+) channel opener BRL34915 and a specific K+ ionophore valinomycin on vasoconstriction induced by endothelin (ET) were compared with those of calcium (Ca2+) channel blockers, nicardipine and verapamil, using helical strips from rat thoracic aorta. ET induced potent and persistent contraction in control solution and similar but smaller contraction in Ca2+-free solution. BRL34915 and valinomycin inhibited the ET-induced contraction dose-dependently in control solution, but not in Ca2+-free solution. The ET-induced contraction was also inhibited by nicardipine and verapamil, though less strongly. On the other hand, high K+ (35 mM)-induced vasoconstriction was strongly inhibited by nicardipine and verapamil, but not by BRL34915 or valinomycin. These results support the idea that the extracellular Ca2+-dependent component of the ET-induced contraction may be mediated by Ca2+ influx by a route other than voltage-dependent Ca2+-channels.     

Dihydropyridine Modulation of Voltage-Activated Calcium Channels in PC12 Cells: Effect of Pertussis Toxin Pretreatment

In this study, we report the effect of pertussis toxin pretreatment on dihydropyridine modulation of voltage-sensitive calcium channels in PC12 cells. The rise in intracellular calcium concentration caused by potassium depolarization is not affected significantly by pertussis toxin pretreatment. Nicardipine, a dihydropyridine derivative, added either before or after potassium-induced depolarization, reduces the resultant elevation in cytosolic calcium level both in control and in pertussis toxin-treated cells. The dihydropyridine agonist Bay K 8644, when added before potassium, is able to enhance the potassium-induced spike of cytosolic calcium levels, an effect significantly reduced by pertussis toxin pretreatment. Moreover, the addition of Bay K 8644 after potassium holds the intracellular calcium concentration at a cytosolic sustained level during the slow inactivating phase of depolarization. This effect of Bay K 8644 is inhibited by nicardipine. Pertussis toxin pretreatment slightly weakens the effect of Bay K 8644 when added after potassium-induced depolarization, whereas it significantly reduces the nicardipine inhibition of cytosolic calcium rise stimulated by potassium and Bay K 8644, but not by potassium alone. In conclusion, our findings suggest that a pertussis toxin-sensitive guanine nucleotide regulatory protein could be involved in the interaction between dihydropyridine derivatives and voltage-dependent calcium channels.     

Clinical potency of calcium channel blockers in asthma may be related to their inhibition of receptor-mediated phasic responses in vitro.

The calcium channel blockers (CCB) have been clinically effective in exercise-induced asthma. The completeness of protection with the CCB might be related specifically to inhibition of Ca2+ influx or release. To examine this hypothesis, the rank order of potency of inhibition of the CCB, nicardipine, diltiazem and verapamil on the steady-state and kinetic parameters of the phasic and tonic responses to the muscarinic receptor agonist carbachol (10 microM) and KCl (40 mM) in the intact isolated guinea-pig trachea was determined. The Ca2+ channel agonist Bay K 8644 was also examined for its effects on intracellular Ca2+. Nicardipine abolished the KCl response at both 0.1 microM and 1 microM concentrations. The amplitude of the KCl response was inhibited equally by 1 microM diltiazem (61% inhibition) and 1 microM verapamil (68% inhibition). The rate constant of onset of the KCl response was similarly inhibited 60% by diltiazem and 66% by verapamil. Nicardipine abolished the carbachol phasic response at the 1 microM concentration. The amplitude of the phasic response was inhibited equally by 0.1 microM nicardipine (61.3% inhibition), 1 microM diltiazem (64.5% inhibition) and 1 microM verapamil (71% inhibition). The rate constant of decay of the phasic response was inhibited equally by 0.1 microM nicardipine (43% inhibition) and 1 microM diltiazem (29% inhibition). The rate constant of onset of the phasic response was unaffected by nicardipine, diltiazem and verapamil. Only 1 microM nicardipine inhibited the amplitude and rate constant of onset of the tonic response. The only effect of Bay K 8644 (1 microM) was to increase the phasic response amplitude. The CCB demonstrate a similar order of potency for inhibition of the phasic responses and clinical efficacy of the CCB in exercise-induced asthma (nicardipine > verapamil > diltiazem).(ABSTRACT TRUNCATED AT 250 WORDS)     

Plasmodium chabaudi: in vivo effects of Ca2+ antagonists on chloroquine-resistant and chloroquine-sensitive parasites

The effects of Ca2+ antagonists, verapamil, nicardipine, and diltiazem, on susceptibility to chloroquine were examined in mice infected with chloroquine-sensitive and chloroquine-resistant lines of Plasmodium chabaudi. In mice that received no chloroquine, daily injections of 50 mg/kg of verapamil, nicardipine, or diltiazem did not affect the growth of both sensitive and resistant parasites. When mice were injected daily with verapamil plus 2 to 3 mg/kg chloroquine, the chloroquine-sensitive parasite became more susceptible to chloroquine than the parasite in mice given chloroquine alone. On the other hand, in mice infected with chloroquine-resistant parasites, verapamil severely suppressed the growth of the parasite when accompanied by daily injections of 2 to 3 mg/kg of chloroquine, at which doses resistant parasites grew steadily in the absence of verapamil, indicating reversal of chloroquine resistance. This reversal was dose-dependent between 5 and 50 mg/kg of verapamil. Daily injections of nicardipine or diltiazem at 50 mg/kg also reversed resistance to chloroquine in resistant parasites. These results indicate that Ca2+ antagonists increase the susceptibility to chloroquine in a sensitive line of P. chabaudi and reverse chloroquine resistance in a resistant line.     

Arachidonic acid stimulates phosphoinositide metabolism and catecholamine release from bovine adrenal chromaffin cells

Arachidonic acid (AA) evoked a dose-dependent increase in the accumulation of inositol phosphates in cultured bovine adrenal chromaffin cells, and this effect was specific for AA. AA also induced a rise in [Ca2+]i, but this rise was markedly reduced by removal of extracellular Ca2+. AA-induced accumulation of inositol phosphates was absolutely dependent on extracellular Ca2+, and nicardipine and nifedine partially reduced it but verapamil had no effect. Moreover, AA dose-dependently stimulated catecholamine release from chromaffin cells in the presence of ouabain, and this effect was specific for AA. AA-induced catecholamine release in the presence of ouabain was also inhibited by nicardipine and nifedipine but not by verapamil. Furthermore, the phospholipase C inhibitor neomycin inhibited the release. These results taken together suggest that AA stimulates catecholamine release in the presence of ouabain by stimulation of phosphoinositide metabolism in a Ca2(+)-dependent manner.     

Photoalteration of calcium channel blockade in the cardiac Purkinje fiber.

Organic compounds that block calcium channel current (calcium antagonists) are important tools for the characterization of this channel. However, the practically irreversible nature of this block restricts the usefulness of this group of drugs. In this paper, we investigate the influence of light on calcium channel blockade by several organic compounds. Our results show that inhibition of calcium channel current by two dihydropyridine derivatives that contain an o-nitro moiety (nisoldipine and nifedipine) can be rapidly reversed by illumination. The energy range important to this reaction is for light wavelengths between 320 and 450 nm. Calcium channel inhibition by two other dihydropyridine derivatives (nicardipine and nitrendipine) as well as by D600, is not modulated by illumination. These results indicate that the photosensitivity of certain dihydropyridine calcium channel blockers make these compounds useful as reversible blockers of this channel.     

Transient changes in blood pressure during spontaneous deep breaths in rats with sinoaortic deafferentation.

Sinoaortic deafferentation (SAD) in rats produces moderate increases in mean arterial pressure (MAP) along with a large augmentation of arterial pressure lability (APL). The mechanisms generating this APL are incompletely understood. To study the possible influence of breathing activity on APL in conscious SAD rats, we simultaneously recorded pulmonary ventilation and arterial blood pressure. The general pattern of pulmonary ventilation was the same in normal, sham-operated, and SAD rats. In all groups single large tidal volumes were regularly interposed in 1- to 2-min periods of shallower breathing. In SAD rats these single large inspirations were consistently accompanied by substantial and abrupt reductions of MAP, whereas this effect was markedly smaller or absent in normal and sham-operated rats. The data reflect the lack of fast moment-to-moment control of arterial pressure normally exerted by the aortic and carotid baroreceptors. In this context, effects of ventilatory changes must be considered along with humoral and neurogenic factors to explain APL after SAD.     

Cardiorenal reflexes do not attenuate the renal effects of infused atriopeptin in conscious dogs.

Low-dose infusions of atriopeptin produce only a modest diuresis and natriuresis. However, these infusions also decrease atrial pressures, a change that has been postulated to elicit an antidiuretic and antinatriuretic reflex from cardiac receptors and thereby to attenuate the direct renal effects of atriopeptin. To determine whether the renal effects of intravenously administered atriopeptin might be attenuated by a cardiorenal reflex, we infused alpha-human atrial natriuretic peptide (alpha-hANP) into cardiac-denervated and sham-operated (normal) conscious dogs. Following a control period, alpha-hANP was infused into each dog at 12.5, 25, or 50 ng.kg-1.min-1 for 1 hr. Infusion of alpha-hANP at 50 ng.kg-1.min-1 produced similar decreases in left atrial pressure in both normal and cardiac-denervated dogs (peak changes, -1.6 +/- 0.8 vs -2.4 +/- 0.9 mm Hg, respectively). Increases in urine flow (peak changes, 0.13 +/- 0.05 vs 0.20 +/- 0.06 ml/min) and sodium excretion (peak changes, 56 +/- 22 vs 70 +/- 11 microEq/min) also were not different between groups. The lower doses of alpha-hANP also elicited renal and hemodynamic responses in the cardiac-denervated dogs that did not differ significantly from those in the normal dogs. These data indicate that the diuresis and natriuresis elicited by intravenously administered alpha-hANP are not attenuated by a cardiorenal reflex in conscious dogs.     

Arterial baroreflex alters strength and mechanisms of muscle metaboreflex during dynamic exercise

Previous studies showed that the arterial baroreflex opposes the pressor response mediated by muscle metaboreflex activation during mild dynamic exercise. However, no studies have investigated the mechanisms contributing to metaboreflex-mediated pressor responses during dynamic exercise after arterial baroreceptor denervation. Therefore, we investigated the contribution of cardiac output (CO) and peripheral vasoconstriction in mediating the pressor response to graded reductions in hindlimb perfusion in conscious, chronically instrumented dogs before and after sinoaortic denervation (SAD) during mild and moderate exercise. In control experiments, the metaboreflex pressor responses were mediated via increases in CO. After SAD, the metaboreflex pressor responses were significantly greater and significantly smaller increases in CO occurred. During control experiments, nonischemic vascular conductance (NIVC) did not change with muscle metaboreflex activation, whereas after SAD NIVC significantly decreased with metaboreflex activation; thus SAD shifted the mechanisms of the muscle metaboreflex from mainly increases in CO to combined cardiac and peripheral vasoconstrictor responses. We conclude that the major mechanism by which the arterial baroreflex buffers the muscle metaboreflex is inhibition of metaboreflex-mediated peripheral vasoconstriction.