Functional and histochemical characterization of a uterine adrenergic denervation process in pregnant rats

The time course of pregnancy-induced changes in the contractile responses of isolated uterine rings and sympathetic innervation pattern were studied using electric field stimulation and histofluorescence techniques, respectively, in intact and 6-hydroxydopamine-treated rats. Neurally mediated contractions elicited by field stimulation (0.6 msec, 1-70 Hz, 40 V) were measured in uterine preparations obtained from nonpregnant, 6-hydroxydopamine-treated and 5-, 10-, 15-, 18-, and 22-day (term) pregnant rats. At all frequencies, the amplitudes of contractions were highest in nonpregnant uteri. Stimulation at 1-2.5 Hz evoked contractions in 10-day pregnant uteri but failed to cause contractions on Day 5 and from Day 15 onward. In uterine preparations obtained from term and from 6-hydroxydopamine-treated rats, contractions could not be evoked by stimulation at 1-20 Hz. Fluorescence histochemistry of uterine adrenergic nerves revealed rich perivascular and myometrial innervation in nonpregnant and in pregnant rats through Day 10. Degeneration and loss of adrenergic nerve fibers was apparent by Day 15, and fluorescent myometrial and perivascular nerves were practically absent by Day 22. These findings demonstrate a progressive, frequency-related reduction of nerve-mediated uterine contractions beginning in midterm pregnancy, in parallel with a gradual loss of adrenergic nerve fibers. Pregnancy-induced nerve degeneration may promote the development of nonsynaptic alpha-adrenergic uterine contractile activity towards term. The reduced responsiveness of uterine smooth muscle to electric field stimulation in early pregnancy appears to be unrelated to alterations in uterine innervation but may be related to changes associated with implantation.     

Autonomic innervation of the circular and longitudinal layers in swine myometrium.

Experiments were conducted on uteri excised from 44 gilts to clarify the autonomic innervation of the longitudinal (LM) and circular muscle (CM) layers of the myometrium. Functionally and biochemically, the two layers differed markedly in their reaction to transmitters. On transmural nerve stimulation (TMS) of isolated LM strips, relaxation was elicited and spontaneous contraction was inhibited in proportion to the electrical frequency imparted. Although the relaxation was accompanied by preliminary contraction in half the LM preparations tested, the relaxation phase predominated in all the LM strips. Relaxation was sensitive to carteolol (beta-blocker) and to guanethidine (adrenergic neuron blocker), whereas the contractile response in LM was sensitive to phentolamine (alpha-adrenergic antagonist). In the CM strips, contraction resulted from TMS, and though not responsive to hexamethonium, the contractions were enhanced by neostigmine and abolished by atropine. The amount of norepinephrine (NE) and the intensity of dopamine beta-hydroxylase activity were about 2.5 times greater in LM than in CM. Conversely, choline acetyltransferase activity, associated exclusively with cholinergic nerves, was about 8 times more intense in the CM. In line with the TMS responses, alpha-receptor-mediated contractions initiated by NE were enabled exclusively in the LM. Furthermore, beta-receptor-mediated inhibition elicited by isoproterenol was also paramount in the LM. We conclude that there are layer-specific variations in the functional innervation of the myometrium of the nulliparous pig uterus such that CM layer is primarily endowed with cholinergic innervation and the LM layer with adrenergic innervation.     

Development and properties of the secretory response in rat sweat glands: relationship to the induction of cholinergic function in sweat gland innervation

Previous studies suggest that the sympathetic innervation of the sweat glands in the rat is initially noradrenergic and during development undergoes a transition in neurotransmitter phenotype to become cholinergic. To characterize this system and its development further, we have examined the adrenergic and cholinergic components of the secretory response in adult and immature rats and have studied the onset of sweating in the plantar sweat glands of developing rats. Stimulation of the sciatic nerve in adult rats elicited a secretory response which was completely blocked by the cholinergic antagonist, atropine, and was unaffected by adrenergic antagonists, indicating that nerve-evoked secretion was cholinergic. In adult rats, the sweat glands were quite sensitive to cholinergic agonists. In addition to acetylcholine, the mature sweat gland innervation contains vasoactive intestinal peptide (VIP). In some rats, the injection of VIP alone elicited a secretory response which was blocked by atropine, suggesting that the response to VIP was mediated cholinergically. In contrast to cholinergic agonists, the glands responded relatively infrequently and with reduced volumes of sweat to the alpha- and beta-adrenergic agonists 6-fluoronorepinephrine and isoproterenol. However, when VIP, which is a potent vasodilator, was simultaneously injected with adrenergic agonists, glands in many of the injected footpads exhibited a secretory response. The response to adrenergic agonists in combination with VIP was reduced by atropine and by phentolamine plus propranolol, but was blocked completely only by a combination of the three antagonists, indicating that both adrenergic and cholinergic mechanisms were involved. In immature rats, sweating evoked by nerve stimulation first appeared at 14 days of age in 25% of the rats tested. Both the percentage of rats sweating and the number of active glands increased rapidly. At 16 days, 50% of the rats tested exhibited some active glands, and by 21 days all rats tested exhibited a secretory response. In 16-day-old rats, nerve-evoked sweating was almost completely inhibited by local injection of 1 microM atropine, but was unaffected by phentolamine and propranolol in concentrations up to 10 microM. Similarly, the glands were sensitive to 10 microM muscarine, but they exhibited no secretory response to the alpha-adrenergic agonists, clonidine and 6-fluoronorepinephrine, nor to the beta-adrenergic agonist, isoproterenol, at concentrations up to 50 microM. The simultaneous injection of VIP with adrenergic agonists did not reveal an adrenergically mediated secretory response in 16-day-old animals.(ABSTRACT TRUNCATED AT 400 WORDS)     

Human female bladder and its noncholinergic contractile function

The response of human female detrusor muscle to field stimulation at varying voltages, durations, and frequencies was studied in vitro. In addition, the effects of adrenergic and cholinergic agonists and antagonists, and various nerve toxins were studied. Beta-adrenergic receptors were found in detrusor muscle but no significant adrenergic innervation was seen; no alpha-adrenergic receptors were seen. Atropine, scorpion venom, tetrodotoxin, beta bungarotoxin and hemicholinium were found to inhibit bladder contraction at short-pulse durations and low frequencies by approximately 50%. Black widow spider venom was seen to abolish bladder contractions entirely. It is concluded that acetylcholine is the neurotransmitter responsible for approximately 50% of bladder contraction. The remaining 50% would seem to be noncholinergic and not dependent on fast sodium channels for transmission of excitation, but would seem to be due to a structure with a short-membrane time constant, such as nerve, and is sensitive to black widow spider venom.     

Autonomic response characteristics of porcine airway smooth muscle in vivo

We studied the autonomic response characteristics of airways in 65 swine in vivo. Tracheal smooth muscle response was measured isometrically in situ; bronchial response was measured simultaneously as change in airway resistance and dynamic compliance. To determine the optimal resting length at which maximal tracheal contraction was obtained, length-tension studies were generated in four animals using maximal electrical stimulation of the vagus nerves determined from stimulus-response characteristics in eight other swine. Pharmacological studies were performed in 25 animals to determine the relative potency and intrinsic activity of agonists (acetylcholine greater than histamine much greater than norepinephrine) causing contraction of trachea and bronchial airways. In 13 swine, the effects of autonomic stimulation were studied by intravenous administration of dimethylphenylpiperazinium (DMPP) after muscarinic blockade with 1.5 mg/kg iv atropine. Tracheal contraction caused by topical application of 3.4 X 10(-4) mol histamine (13.4 +/- 1.54 g/cm) was 96 +/- 7.2% blocked by 25 micrograms/kg iv DMPP in adrenal-intact animals; minimal relaxation was demonstrated in adrenalectomized animals, indicating absence of substantial sympathetic innervation to porcine trachea. Nonadrenergic innervation was not demonstrated. After beta-adrenergic blockade, sympathetic stimulation caused alpha-adrenergic contraction in bronchial airways but not in trachea. These data define the unique response characteristics of the airways of swine and demonstrate their utility for acute experimental study of airway responses in vivo.     

Neural control of contraction in isolated submucosal gland from feline trachea

To determine the autonomic innervation to myoepithelial cells of submucosal gland, we applied electrical field stimulation (FS) to the intrinsic nerves in isolated submucosal glands from feline tracheae. FS induced contraction that was voltage or frequency dependent and abolished by pretreatment with tetrodotoxin. DMPP (1,1-dimethyl-4-phenylpiperazinium iodide) did not produce any significant contraction, and pretreatment with hexamethonium did not alter the response to FS. Atropine inhibited the contractile response to FS and neostigmine augmented the response to FS. Serotonin also augmented the response to FS, whereas the response to methacholine remained unchanged in the presence of serotonin. Phentolamine reduced the response to FS by 15% of control, whereas propranolol induced no significant changes in the response to FS. No significant inhibitory responses were observed by FS. Our findings indicate that the contraction of tracheal submucosal glands is mediated mainly by cholinergic nerves via muscarinic receptors and in small part by adrenergic nerves via alpha-receptors, and serotonin potentiates the contractile response to FS at the postganglionic nerve.     

Physiological antagonism caused by adrenergic stimulation of canine tracheal muscle

We studied the simultaneous alpha- and beta-adrenergic response characteristics of canine tracheal smooth muscle in 398 strips from 67 dogs in vitro. Experiments were performed to determine the effects of beta-adrenergic blockade on the expression of the alpha-adrenoceptor contractile responses elicited by norepinephrine (NE), phenylephrine (PE), and clonidine (CLO). Maximal active tension caused by NE increased from 39.1 +/- 27.0 to 241 +/- 75.0 g/cm2 as the concentration of propranolol (PROP) was increased from 10(-6) to 10(-4) M. Augmentation of tracheal smooth muscle contraction caused by PE and CLO was also observed with progressive beta-adrenoceptor blockade; contraction to NE, PE, and CLO was blocked selectively with 3 X 10(-5) M phentolamine (PA) and phenoxybenzamine (PBZ). The beta-adrenergic relaxing properties of the same three agonists were also studied. After alpha-adrenergic blockade with PA or PBZ, all three agonists caused relaxation (NE greater than CLO greater than PE) of methacholine-induced contraction of tracheal smooth muscle that was reversed selectively with PROP. We demonstrate that NE, PE, and CLO cause simultaneous stimulation of both the alpha- and beta-adrenergic receptors in tracheal smooth muscle; the net response elicited is the result of adrenergic physiological antagonism and depends on the relative degree of alpha- and/or beta-adrenoceptor blockade.     

Calcium fluxes in isolated acinar cells from rat parotid. Effect of adrenergic and cholinergic stimulation.

Rat parotid acinar cells dispersed by a combination of enzymatic treatments remain sensitive to adrenergic and cholinergic agonists. Previous studies have implicated Ca2+ in both adrenergic and cholinergic responses. This paper describes the effects of adrenergic and cholinergic stimulation upon 45Ca2+ fluxes in isolated parotid acinar cells. Suspensions of dispersed cells took up 45Ca2+ from the medium. The net rate of isotope influx was increased by the adrenergic agonists epinephrine, norepinephrine, isoproterenol, and phenylephrine, and by the cholinergic agonists acetylcholine and carbamylcholine. In 1 mM Ca2+, epinephrine was capable of increasing the 45Ca2+ influx in 40 min to three times that of resting cells. Isoproterenol, a beta-adrenergic agonist, was only half as effective as epinephrine in stimulating maximal calcium uptake although it was equally effective in stimulating maximal amylase release in the same cells. Experiments with the alpha-adrenergic antagonist phentolamine, the beta-adrenergic antagonist propranolol, and the cholinergic antagonist atropine confirmed that alpha- and beta-adrenergic and cholinergic stimulation each had a direct stimulatory effect on 45Ca2+ uptake. N6,O2'-Dibutyryl adenosine 3':5'-monophosphate also caused some stimulation of net calcium uptake. Direct measurement of Ca2+ efflux indicated that the increased calcium uptake in the presence of epinephrine was not the indirect result of a decrease in efflux. The rates of both basal and epinephrine-stimulated calcium uptake increased with increasing calcium concentration in the medium. Epinephrine had little effect on the rate of calcium uptake at 0.15 mM Ca2+. Although the energy poison NaCN had little effect on the basal rate of calcium uptake, the stimulable component of calcium uptake was inhibited by NaCN at all calcium concentrations tested (0.2 to 4.1 mM).     

Pharmacokinetic analysis of alpha- and beta-adrenoreceptors in the chick embryo amnion

Following a stimulation with acetylcholine, the beta-adrenergic agonists adrenaline (A), noradrenaline (NA), isoproterenol (Iso) and salbutamol (Sal) induced a concentration-dependent decrease in the tone and (or) rate of amnion contraction with EC50 ISO < NA < A < Sal. Metaprolol, a specific beta 1-antagonist, induced a rightward shift in the dose-response curves of Iso, NA and A, whereas beta-antagonist butoxamine was ineffective. pA2 values for beta-antagonists were propranolol 8.3, metoprolol 7.0, butoxamine 5.6. EC50 values of alpha-adrenergic agonists form a sequence: clonidine < NA < methoxamine < phenylephrine. Specific alpha-antagonists yohimbine and idazoxan were found to antagonise competitively the effects of NA. The data obtained characterize the adrenergic receptors mediating stimulation of amniotic contractile activity as alpha 2-adrenergic receptors. Inhibition of contractile receptors in amnion is mainly mediated by beta 1-adrenergic receptor activation.     

Alpha-adrenergic stimulation of sarcolemmal protein phosphorylation and slow responses in intact myocardium

The effects of alpha- and beta-adrenergic stimulation on sarcolemmal protein phosphorylation and contractile slow responses were studied in intact myocardium. Isolated rat ventricles were perfused via the coronary arteries with 32Pi after which membrane vesicles partially enriched in sarcolemma were isolated from individual hearts. Alterations in the sarcolemmal slow inward Ca2+ current were assessed in the 32P-perfused hearts using a contractile slow response model. In this model, Na+ channels were first inactivated by partial depolarization of the hearts in 25 mM K+ after which alterations in Ca2+ channel activity produced by either alpha- or beta-adrenergic agonists could be assessed as restoration of contractions. alpha-Adrenergic stimulation (phenylephrine + propranolol) of the perfused hearts resulted in increased 32P incorporation into a 15-kDa sarcolemmal protein. This protein co-migrated with the 15-kDa sarcolemmal protein phosphorylated in hearts exposed to beta-adrenergic stimulation produced by isoproterenol. beta-Adrenergic stimulation, but not alpha-adrenergic stimulation, also resulted in phosphorylation of the sarcoplasmic reticulum protein, phospholamban. Phosphorylation of the 15-kDa protein in perfused hearts in response to either alpha- or beta-adrenergic stimulation was associated with restoration of contractions, indicative of increases in the slow inward Ca2+ current. Increases in 32P incorporation into the 15-kDa protein preceded restoration of contractions by phenylephrine. Nifedipine abolished the contractile responses to alpha-adrenergic stimulation while having no effect on increases in 15-kDa protein phosphorylation. The effects of alpha-adrenergic stimulation occurred in the absence of increases in cAMP levels. These results suggest that phosphorylation of the 15-kDa protein may be involved in increases in the slow inward current produced by stimulation of either alpha- or beta-adrenergic receptors.     

Effect of prolonged gamma-irradiation on the functional state of heart and its adrenergic regulation at hypothyroidism

The study of heart isolated by Langendorf's method has shown that the prolongated gamma-irradiation of euthyroid rats in 1.0 Gy dose (2.8 x 10(-7) Gy/sec) causes the decrease in contraction ability, myocardium relaxation and functional response of heart to the stimulation of beta-adrenergic receptors, and the increase in myocardium reaction to the stimulation of alpha-adrenergic receptors. The irradiation of hypothyroid animals leads to more significant and long changes in contraction function of heart and its adrenergic regulation.     

Regulation of alpha- and beta-adrenergic receptor subclasses by gonadal steroids in human myometrium

Both alpha- and beta-adrenergic receptors have been identified in the human myometrium by radioligand binding. Both adrenergic receptor subclasses have been shown to mediate the contractile response of the uterus upon catecholamine stimulation: alpha-adrenergic receptors cause uterine contraction while beta-adrenergic receptors induce relaxation. We have identified alpha 1- and alpha 2-adrenergic receptors in myometrial membranes using the newly developed radiolabelled specific antagonists [3H]-prazosin and [3H]-rauwolscine. This enabled us to characterize both receptor subclasses individually. Beta adrenergic receptors were identified using the radiolabelled antagonist (-)-[3H]-dihydroalprenolol. Binding of radioligands to the myometrial membrane receptors was rapid, readily reversible, of high affinity and stereoselective. The total number of alpha 1-, alpha 2- and beta-receptors was determined by Scatchard analysis of radioligand saturation binding and the beta/beta 2-receptor ratio was determined by computer analysis of the beta 2-selective antagonist ICI 118 551) (-)-[3H]-dihydroalprenolol competition binding curves. This enabled us to study the regulation of both alpha- and beta-receptor subclasses under various physiological and pharmacological conditions in the human, i.e., during different phases of the menstrual cycle, in postmenopausal women and during depo-progestin (Medroxyprogesterone acetate) therapy. Only the alpha 2- and beta 1-adrenergic receptor concentrations were found to be subjected to gonadal steroid regulation. The number of alpha 2- and beta 1-adrenergic receptors increased concomitantly with circulating plasma oestradiol levels. This effect was counteracted by progesterone. The number of alpha 1- and beta 2-adrenergic receptors was unaffected by the gonadal steroid environment. These results are an example of the heteroregulation of membrane receptors by oestrogens and progesterone and cast new light on the regulatory mechanisms involved in uterine contractility in the human.     

Comparative effect of atropine on the adrenergic and muscarinic stimulation of phospholipid 32P labelling in isolated parotid cells: atropine, a possible blocker of alpha-adrenergic receptors

The inhibitory effect of atropine on phospholipid 32P labelling stimulated by muscarinic or alpha-adrenergic agonists was studied in isolated parotid cells. Atropine (10(-11) to 10(-4) M) had no effect on phospholipid 32P labelling in unstimulated cells. In contrast, 10(-8) to 10(-7) M atropine provoked a competitive inhibition of the cholinergic stimulation (i.e. this effect was completely wiped out at high agonist concentration). The atropine app. KD for the muscarinic receptor was 5 X 10(-9) M. Moreover, atropine inhibits the adrenergic stimulation of phospholipid 32P labelling by decreasing the efficacity and potency of the adrenergic agonists. The atropine app. KD for the alpha-adrenergic receptor can be estimated at 10(-5) M. This inhibition of alpha-adrenergic stimulation appears to be specific since atropine was without effect on the substance P or beta-adrenergic stimulation. At very low concentration (10(-10) - 10(-9) M) atropine seems to be a modulator (activator) of the muscarinic or adrenergic agonist-receptor complex. From the present data, it is suggested that atropine, besides its classical blocker effect at the muscarinic receptor, at high concentration is a specific alpha-adrenergic antagonist.     

Comparative effect of atropine on the adrenergic and muscarinic stimulation of phospholipid 32P labelling in isolated parotid cells: atropine,a possible blocker of alpha-adrenergic receptors

The inhibitory effect of atropine on phospholipid 32P labelling stimulated by muscarinic or alpha-adrenergic agonists was studied in isolated parotid cells. Atropine (10(-11) to 10(-4) M) had no effect on phospholipid 32P labelling in unstimulated cells. In contrast, 10(-8) to 10(-7) M atropine provoked a competitive inhibition of the cholinergic stimulation (i.e. this effect was completely wiped out at high agonist concentration). The atropine app. KD for the muscarinic receptor was 5 × 10(-9) M. Moreover, atropine inhibits the adrenergic stimulation of phospholipid 32P labelling by decreasing the efficacity and potency of the adrenergic agonists. The atropine app. KD for the alpha-adrenergic receptor can be estimated at 10(-5) M. This inhibition of alpha-adrenergic stimulation appears to be specific since atropine was without effect on the substance P or beta-adrenergic stimulation. At very low concentration (10(-10) — 10(-9) M) atropine seems to be a modulator (activator) of the muscarinic or adrenergic agonist-receptor complex. From the present data, it is suggested that atropine, besides its classical blocker effect at the muscarinic receptor, at high concentration is a specific alpha-adrenergic antagonist.     

Response of the isolated rat iris sphincter to cholinergic and adrenergic agents and electrical stimulation

In isolated rat iris sphincter muscle, there has been no attempt to measure mechanical tension changes, because of the small size of the preparation. In this study, responses of the isolated rat iris sphincter to some agents and electrical stimulation were examined. Acetylcholine and electrical stimulation produced powerful contractions of the iris sphincter. These contractile responses were suppressed by atropine and enhanced by physostigmine. 10 μM norepinephrine induced a weak contraction of the sphincter muscle and 1 mM isoproterenol induced a very weak relaxation. These responses were antagonized by phentolamine and propranolol, respectively. In the presence of 0.1 μM atropine, electrical stimulation produced a weak alpha-adrenergic contraction and a very weak beta-adrenergic relaxation. Electrically induced responses were abolished by tetrodotoxin. In conclusion, in the rat iris sphincter, powerful contraction is due to the activation of muscarinic receptors, and that there are weak alpha-adrenergic contraction and weak beta-adrenergic relaxation. Thus in rats, muscarinic contraction of the sphincter muscle plays major role in the regulation of pupil diameter.     

The cardiac innervation of a marsupial heterotherm,the fat-tailed dunnart (Sminthopsis crassicaudata)

This study investigated the pattern of autonomic innervation of the heart of the fat-tailed dunnart (Sminthopsis crassicaudata) using isolated cardiac preparations. While the pattern of autonomic innervation of the atria was consistent with that found in other mammals, the ventricles displayed an unusual pattern of mammalian cardiac innervation. Transmural stimulation of the intramural nerves of isolated right ventricular preparations caused a decrease in the force of contraction of 46.8+/-3.2% followed by a rebound increase in the force of contraction beyond basal levels of 40.9+/-6.9%. These responses could be blocked independently by the application of the muscarinic receptor antagonist hyoscine and beta-adrenoreceptor antagonist propranolol respectively and could also be mimicked by the application of the agonists acetylcholine (Ach) and noradrenaline (NA). These findings indicated the presence of a functional cholinergic innervation of the ventricles that was capable of reducing the force of contraction below basal levels. This pattern of innervation has only been found previously in one other mammal, the bent-winged bat (Miniopterus schreibersii). Given that both of these species are heterotherms, it is possible that such a pattern of innervation may relate to the control of cardiac output during torpor. These findings are the first that demonstrate the homogeneity of a physiological control mechanism in a so-called 'shallow, daily torpidator' (S. crassicaudata) and a 'deep hibernator' (M. schreibersii) that is absent in mammalian homeotherms. These findings are consistent with recent work suggesting that there may be little difference between these types of heterothermy.     

Multiple Adrenergic Receptor Subtypes Controlling Cyclic AMP Formation: Comparison of Brain Slices and Primary Neuronal and Glial Cultures

The adrenergic receptor subtypes involved in cyclic AMP responses to norepinephrine (NE) were compared between slices of rat cerebral cortex and primary neuronal and glial cultures from rat brain. In neuronal cultures, NE and the beta-adrenergic receptor agonist isoproterenol (ISO) caused similar increases in cyclic AMP, which were not altered by the alpha-adrenergic receptor antagonist phentolamine. In glial cultures, NE caused a much smaller cyclic AMP response than did ISO, and this difference was reversed by alpha-adrenergic receptor antagonists (phentolamine greater than yohimbine greater than prazosin). alpha 2-Adrenergic receptor agonists partially inhibited the ISO response in glial cultures to a level similar to that observed with NE alone (clonidine = UK 14,304 greater than NE greater than 6-fluoro-NE greater than epinephrine). In slices from cerebral cortex, NE caused a much larger increase in cyclic AMP than did ISO, and this difference was reversed by alpha-adrenergic receptor antagonists with a different order of potency (prazosin greater than phentolamine greater than yohimbine). alpha 1-Adrenergic receptor agonists potentiated the response to ISO to a level similar to that observed with NE alone (epinephrine = NE greater than phenylephrine greater than 6-fluoro-NE greater than methoxamine). In all three tissue preparations, large responses to both alpha 1-receptor activation (increases in inositol phosphate accumulation) and alpha 2-receptor activation (decreases in forskolin-stimulated cyclic AMP accumulation) were observed. These data indicate that all of the major adrenergic receptor subtypes (beta, alpha 1, alpha 2) are present in each tissue preparation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibition of the lipolytic action of beta-adrenergic agonists in human adipocytes by alpha-adrenergic agonists

The aim of this study was to define the role of the alpha-adrenergic receptor in the regulation of lipolysis by human adipocytes. Glycerol production by isolated human adipocytes was stimulated by the pure beta-adrenergic agonist isoproterenol in a dose-dependent fashion. This stimulation of lipolysis was inhibited by the alpha-adrenergic agonists methoxamine, phenylephrine, and clonidine. Epinephrine-stimulated lipolysis was potentiated by the alpha-adrenergic antagonists, dihydroergocryptine, phentolamine, phenoxybenzamine, and yohimbine. Whereas the attenuation of beta-adrenergic agonist-stimulated lipolysis by alpha-adrenergic agonists was reversed completely by the alpha 2-adrenergic antagonist yohimbine, the alpha 1-antagonist prazosin did not reverse such attenuation. It is concluded that alpha-adrenergic agonists act as antilipolytic agents in human adipocytes and that this action may result from the interaction of these compounds with a population of alpha 2-adrenergic receptors.     

The adrenergic regulation of the cardiovascular system in the South American rattlesnake, Crotalus durissus

The present study investigates adrenergic regulation of the systemic and pulmonary circulations of the anaesthetised South American rattlesnake, Crotalus durissus. Haemodynamic measurements were made following bolus injections of adrenaline and adrenergic antagonists administered through a systemic arterial catheter. Adrenaline caused a marked systemic vasoconstriction that was abolished by phentolamine, indicating this response was mediated through alpha-adrenergic receptors. Injection of phentolamine gave rise to a pronounced vasodilatation (systemic conductance (G(sys)) more than doubled), while injection of propranolol caused a systemic vasoconstriction, pointing to a potent alpha-adrenergic, and a weaker beta-adrenergic tone in the systemic vasculature of Crotalus. Overall, the pulmonary vasculature was far less responsive to adrenergic stimulation than the systemic circulation. Adrenaline caused a small but non-significant pulmonary vasodilatation and there was tendency of reducing this dilatation after either phentolamine or propranolol. Injection of phentolamine increased pulmonary conductance (G(pul)), while injection of propranolol produced a small pulmonary constriction, indicating that alpha-adrenergic and beta-adrenergic receptors contribute to a basal regulation of the pulmonary vasculature. Our results suggest adrenergic regulation of the systemic vasculature, rather than the pulmonary, may be an important factor in the development of intracardiac shunts.