Vascular responses after alpha adrenergic receptor blockade: I. Responses of capacitance and resistance vessels to norepinephrine in man.

Experiments were done to test the hypothesis that alpha receptor blockers antagonize more effectively venous than arterial responses to norepinephrine in man.Systemic arterial blood pressure, venous pressure in the forearm, blood flow through the forearm, and the volume of the forearm at a venous pressure of 30 mm Hg were measured using pressure transducers and a mercury strain-gauge plethysmograph. Infusions of norepinephrine into the brachial artery reduced forearm blood flow and venous distensibility without changing arterial pressure. After intraarterial infusion of phentolamine the decrease in venous distensibility during administration of norepinephrine was blocked almost completely whereas the decrease in blood flow through the forearm was not altered.The results indicate that alpha adrenergic receptor blockade can antagonize constriction of capacitance vessels more effectively than constriction of resistance vessels.

Idiopathic postural hypotension: physiologic observations and report of a new mode of therapy.

Two patients with severe postural hypotension associated with upper motor neuron and cerebellar impairment (Shy-Drager syndrome) have been studied. Head-up tilt and lower body negative pressure application caused marked falls in arterial pressure; in one patient, paradoxical vasodilatation was observed. Ice application did not increase arterial pressure or calculated forearm vascular resistance. Intravenous atropine in one patient increased heart rate by 18 beats per min, a cardioacceleratory response similar to exhausting recumbent exercise in that patient. 24 hr urinary catecholamine excretion was low, but aldosterone secretory rate was normal in the more severely afflicted patient. A prolonged elevation of plasma renin activity was noted when post-tilt hypertension occurred. When head-up tilt was not followed by this hypertensive period, plasma renin activity response to tilting was normal. Intra-arterial norepinephrine and tyramine both elicited a vasoconstrictor response. Intra-arterial infusions of norepinephrine and tyramine were repeated after administration of the monoamine oxidase inhibitor tranylcypromine. Norepinephrine was potentiated 4.1- and 0.5-fold in the two patients; tyramine was potentiated 3.7-and 1.1-fold in the two patients, respectively. A therapeutic program of tranylcypromine and tyramine (in the form of cheddar cheese) resulted in substantial clinical improvement. It is concluded that in at least some patients with idiopathic postural hypotension, norepinephrine is present in postganglionic sympathetic fibers. A therapeutic program of tyramine and a monoamine oxidase inhibitor may be of value when more conventional modes of therapy fail.

Comparison of effects of deoxycorticosterone and dexamethasone on cardiovascular responses to norepinephrine.

Cardiovascular responses to graded iv infusions of norepinephrine were observed in 24 dogs that had been treated for 1 week with either placebo, dexamethasone, or deoxycorticosterone. Eight dogs served as control and received daily iv injections of placebo; eight dogs received the mineralocorticoid, deoxycorticosterone; and eight received the glucocorticoid, dexamethasone. The three groups did not differ with respect to base-line hemodynamic variables either before administration of norepinephrine or after autonomic reflexes had been inhibited by ganglionic blockade. Comparisons of the three groups' hemodynamic responses to norepinephrine were made both before and after ganglionic blockade with the parallel line bioassay as a statistical test. Dogs given deoxycorticosterone had much greater increases in mean arterial pressure and peripheral resistance with norepinephrine than did dogs given dexamethasone or placebo. Dogs given dexamethasone had slightly greater increases in mean arterial pressure than did dogs given placebo; changes in peripheral resistance were similar in the two groups. The augmented response of mean arterial pressure was apparent only after ganglionic blockade in the dexamethasone group. The vascular effects of norepinephrine, therefore, were markedly augmented by treatment with doxycorticosterone and only slightly augmented by treatment with dexamethasone. The effect of norepinephrine on mean right atrial pressure was augmented in both groups treated with steroid before hexamethonium but only in the group treated with dexamethasone after hexamethonium. The results indicate that deoxycorticosterone and dexamethasone have different qualitative and quantitative effects on circulatory responses to norepinephrine.

Interaction of baroreceptor and chemoreceptor reflexes. Modulation of the chemoreceptor reflex by changes in baroreceptor activity.

The purpose of this study was to determine whether the level of arterial pressure and degree of baroreceptor activation affect responses to stimulation of chemoreceptors. Chemoreceptors were stimulated by injecting nicotine into the common carotid artery of anesthetized and paralyzed dogs. Responses were observed in the innervated gracilis muscle, perfused at constant flow while perfusion pressure was measured. Arterial pressure was lowered by bleeding the animals and raised by transient occlusion of the descending aorta. Vasoconstrictor responses to stimulation of chemoreceptors were enhanced by hypotension and inhibited by elevation of arterial pressure. Potentiation of the chemoreceptor reflex by hemorrhagic hypotension was not the result of altered vascular resistance in the gracilis muscle, sensitization of chemoreceptors by catecholamines or acidosis, or changes in cerebral perfusion pressure. Additional studies were done in which we excluded the possibility that the changes resulted from direct effects of changes in arterial pressure on chemoreceptors. Both carotid bifurcations were isolated and perfused. On one side, pressure was raised to stimulate the carotid sinus baroreceptors. On the other side, the carotid body chemoreceptors were stimulated by nicotine or by hypoxic and hypercapnic blood. Activation of baroreceptors on one side attenuated the vasoconstrictor response to chemoreceptor stimulation on the other side. This excludes a direct effect of changes in arterial pressure on the chemoreceptors and suggests a central interaction of these reflexes. We conclude that vasoconstrictor responses to stimulation of chemoreceptors are potentiated by hypotension and inhibited by transient hypertension. These effects appear to result at least in part from a central interaction of chemoreceptor and baroreceptor reflexes.

Reflex vascular responses to left ventricular outflow obstruction and activation of ventricular baroreceptors in dogs.

Reflex vascular responses to acute left ventricular outflow obstruction were studied in anesthetized dogs. The studies were done to compare the effects of activation of ventricular baroreceptors on vascular resistance in skeletal muscle (gracilis muscle) and skin (hindpaw); to identify afferent and efferent pathways which mediate the reflex vasodilatation; and to assess the relative contribution of ventricular baroreceptors and baroreceptors in left atrium and pulmonary vessels in responses to left ventricular outflow obstruction. The gracilis artery and the cranial tibial artery to the paw were perfused separately at constant flow. Changes in perfusion pressure to each bed reflected changes in vascular resistance. Outflow obstruction was produced by inflating a balloon in the left ventricular outflow tract for 15 s while pressures in the left ventricle and aortic arch were measured. Inflation of the balloon increased left ventricular pressure and decreased pressure in the aortic arch. Low and high levels of obstruction produced dilator responses averaging -5+/-3 (SE) and -42+/-11 mm Hg in muscle and -1+/-1 and -3+/-2 mm Hg in paw. Denervation, phentolamine, and glyceryltrinitrate caused greater dilatation in paw than did left ventricular outflow obstruction. This indicates that dilator responses in the paw were not limited by a low level of resting neurogenic constrictor tone or by a negligible dilator capacity of these vessels. Obstruction to left ventricular inflow increased left atrial pressure, but did not cause reflex vasodilatation. This suggests that low pressure baroreceptors in atria or pulmonary vessels did not contribute to vasodilator responses to left ventricular outflow obstruction. Vasodilator responses to outflow obstruction were blocked by bilateral vagotomy, sectioning the sciatic and obturator nerves, and administration of phentolamine, but were not decreased by atropine or tripelennamine. The results indicate that activation of left ventricular baroreceptors produces striking vasodilatation in skeletal muscle, but only slight vasodilatation in skin. The data suggest that the difference in dilator responses in the two beds results from greater withdrawal of adrenergic constrictor tone to skeletal muscle than to skin. Activation of sympathetic cholinergic or histaminergic dilator pathways does not contribute to the dilatation.

Impaired reflex vasoconstriction in chronically hypoxemic patients.

Acute hypoxia impairs vasoconstrictor responses in normal men. The present study was done to determine whether reflex vasoconstriction is impaired in chronically hypoxemic patients and whether correction of hypoxemia in these patients improves their cardiovascular reflexes. In eight chronically hypoxemic patients, arterial P(O2) was increased from an average of 45 mm Hg while breathing room air to 161 mm Hg while breathing 40-100% oxygen, with minimal changes in arterial P(CO2) or pH. Correction of hypoxemia did not cause changes in resting arterial pressure or in forearm vascular resistance, but it caused a small increase in resting heart rate. Reflex responses to lower body negative pressure, which causes pooling of blood in the lower part of the body, were observed. When the patients were hypoxemic, lower body negative pressure caused a fall in arterial pressure, slight constriction of forearm vessels, and a small increase in heart rate. When hypoxemia was corrected, the same intervention caused marked vasoconstriction and a greater increase in heart rate, and there was no decrease in arterial pressure. The results indicate that reflex vasoconstrictor responses are depressed in chronic hypoxemia, indicating that adaptive mechanisms which occur in chronic hypoxemia do not include preservation of sympathetic reflexes.

Regulation of blood flow in Paget's disease of bone.

Previous studies have demonstrated an increase in blood flow to extremities involved by Paget's disease of bone. It has been assumed that the increase in blood flow is through bone, but warmth of skin overlying Pagetic bone suggests that cutaneous blood flow might be increased. In three patients with Paget's disease involving one extremity, we compared blood flow in "Pagetic" extremities with flow in the contralateral normal extremities. Resting blood flow (measured with water plethysmographs) was 14.2plus or minus2.9 (meanplus or minusSE) ml/min times 100 ml extremity in the Pagetic limbs. The contribution of cutaneous flow to the increase in extremity blood flow was evaluated with epinephrine iontophoresis, which suppresses flow to skin but not to underlying tissue. Epinephrine iontophoresis of the Pagetic extremities decreased blood flow from 14.2plus or minus2.9 to 3.6plus or minus1.5 ml/min. Local heating (which increases cutaneous flow only) failed to increase blood flow in the Pagetic extremities as much as it did in the normal extremities. This suggests that cutaneous vessels in the Pagetic extremities were already dilated. During heating, blood flow in the normal extremities was similar to resting flow in the Pagetic extremities; this indicates that increases in cutaneous flow could account for most of the increase in total blood flow in the Pagetic extremities. Adrenergic control of blood flow to the Pagetic extremities was compared with that of the normal extremities. Vasoconstrictor responses to reflex stimuli in the Pagetic extremities were reduced; when vasoconstriction occurred it was gradual and sustained after termination of the stimuli, which suggests an exaggerated humoral response but reduced neural response to the stimuli. Intravenous epinephrine produced vasoconstriction in the Pagetic extremities and vasodilatation in the normal extremities. In summary, responses to epinephrine iontophoresis and heating suggest that the increase in blood flow in Pagetic extremities is primarily the result of cutaneous vasodilatation.

Effects of chronic beta-adrenergic blockade on the left ventricular and cardiocyte abnormalities of chronic canine mitral regurgitation.

The mechanism by which beta blockade improves left ventricular dysfunction in various cardiomyopathies has been ascribed to improved contractile function of the myocardium or to improved beta-adrenergic responsiveness. In this study we tested two hypotheses: (a) that chronic beta blockade would improve the left ventricular dysfunction which develops in mitral regurgitation, and (b) that an important mechanism of this effect would be improved innate contractile function of the myocardium. Two groups of six dogs with chronic severe mitral regurgitation were studied. After 3 mo both groups had developed similar and significant left ventricular dysfunction. One group was then gradually beta-blocked while the second group continued to be observed without further intervention. In the group that remained unblocked, contractile function remained depressed. However, in the group that received chronic beta blockade, contractile function improved substantially. The contractility of cardiocytes isolated from the unblocked hearts and then studied in the absence of beta receptor stimulation was extremely depressed. However, contractility of cardiocytes isolated from the beta-blocked ventricles was virtually normal. Consistent with these data, myofibrillar density was much higher, 55 +/- 4% in the beta-blocked group vs. 39 +/- 2% (P < 0.01) in the unblocked group; thus, there were more contractile elements to generate force in the beta-blocked group. We conclude that chronic beta blockade improves left ventricular function in chronic experimental mitral regurgitation. This improvement was associated with an improvement in the innate contractile function of isolated cardiocytes, which in turn is associated with an increase in the number of contractile elements.

Differences in direct effects of adrenergic stimuli on coronary, cutaneous, and muscular vessels.

Direct effects of adrenergic stimuli on coronary vessels in dogs were compared with effects on vessels to skin (hind paw) and skeletal muscle (gracilis muscle) after intravenous administration of practolol (2 mg/kg), a selective myocardial beta receptor blocker which minimized indirect effects of myocardial stimulation on coronary vascular resistance. The left circumflex coronary, cranial tibial, and gracilis arteries were perfused separately but simultaneously at constant flow. Perfusion pressures, left ventricular pressure and dP/dt. and heart rate were recorded. Changes in perfusion pressure to each bed reflected changes in vascular resistance. The direct constrictor effects of sympathetic nerve stimulation, norepinephrine and phenylephrine on coronary vessels were minimal compared with effects on cutaneous and muscular vessels. Subsequent blockade of vascular beta receptors did not augment the constrictor responses. Angiotensin, a nonadrenergic stimulus, produced striking coronary vasoconstriction which exceeded that in skin and approximated that in muscle. These results suggests that there is a paucity of alpha adrenergic receptors in coronary vessels compared to cutaneous and muscular vessels. Direct dilator responses to isoproterenol were similar in coronary and cutaneous vessels, but were greater in muscular vessels. Responses to glyceryl trinitrate, a nonadrenergic dilator, also were greater in skeletal muscle. Therefore, differences in effects of isoproterenol on the three beds may reflect differences in reactivity to dilator stimuli rather than differences in the density of beta receptors. In contrast to norepinephrine, the predominant direct effect of epinephrine on coronary vessels was dilatation mediated through activation of vascular beta receptors. A constrictor effect caused by stimulation of alpha receptors was unmasked by propranolol.Finally, the order of potency of agonists in stimulating coronary vascular beta receptors and the demonstration of selective beta receptor blockade with practolol suggest that beta receptors in coronary vessels resemble those in peripheral vessels more than those in myocardium.

Abnormal vascular responses to exercise in patients with aortic stenosis.

We tested the hypothesis that the normal forearm vasoconstrictor response to leg exercise is inhibited or reversed in patients with aortic stenosis, possibly because of activation of left ventricular baroreceptors. Forearm vascular responses to supine leg exercise were measured in 10 patients with aortic stenosis and in 2 control groups of 6 patients with mitral stenosis and 5 patients without valvular heart disease.Forearm vasoconstriction occurred during exercise in the control groups. In contrast, forearm blood flow increased and forearm vascular resistance did not change in patients with aortic stenosis. In six patients with aortic stenosis and a history of exertional syncope, forearm vasodilatation occurred during the second minute of leg exercise. Inhibition or reversal of forearm vasoconstrictor responses in aortic stenosis was asscociated with significant increases in left ventricular pressure. In three patients with aortic stenosis and exertional syncope, forearm vasodilator responses to exercise changed to vasoconstrictor responses after aortic valve replacement. The results indicate that forearm vasoconstrictor responses to leg exercise are inhibited or reversed in patients with aortic stenosis, possibly because of activation of left ventricular baroreceptors. The observations suggest that reflex vasodilatation resulting from activation of left ventricular baroreceptors may contribute to exertional syncope in patients with aortic stenosis.

Effects of adrenergic stimulation on ventilation in man.

The mechanism by which catecholamines affect ventilation in man is not known. Ventilatory responses to catecholamines were observed in normal subjects before and after adrenergic receptor blockade. Intravenous infusions of norepinephrine and isoproterenol caused significant increases in minute volume and decreases in end-tidal P(Co2) which were blocked by the administration of propranolol, a beta adrenergic receptor blocker. The hyperventilatory response to hypoxia was not altered by propranolol. Intravenous infusion of phenylephrine caused a small but significant decrease in minute volume which was antagonized by phentolamine, an alpha adrenergic receptor blocker. Angiotensin, a nonadrenergic pressor agent, also decreased minute volume significantly.100% oxygen was administered to suppress arterial chemoreceptors. Increases in minute volume and decreases in arterial P(Co2) in response to norepinephrine and isoproterenol were blocked by breathing 100% oxygen. The decrease in minute volume during phenylephrine was not altered by 100% oxygen. THE RESULTS INDICATE THAT: (a) beta adrenergic receptors mediate the hyperventilatory response to norepinephrine and isoproterenol but not to hypoxia. (b) the pressor agents phenylephrine and angiotensin decrease ventilation, and (c) suppression of chemoreceptors blocks the ventilatory response to norepinephrine and isoproterenol but not to phenylephrine. Implications concerning the interaction of adrenergic receptors and chemoreceptors with respect to the hyperventilatory response to catecholamines are discussed.

The role of low pressure baroreceptors in reflex vasoconstrictor responses in man.

Studies were performed on 11 healthy men to evaluate the role of low pressure baroreceptors in the reflex forearm vasoconstrictor responses (plethysmography) to venous pooling produced by lower body negative pressure. Lower body negative pressure (LBNP) at - 5, - 10, - 20, and - 40 mm Hg lowered central venous pressure by 42, 59, 74, and 93%, respectively, and decreased forearm vascular conductance by 24, 29, 34, and 40%, respectively. The decreases in forearm blood flow and conductance during the low levels of venous pooling (LBNP - 5 and - 10 mm Hg) occurred without significant changes in arterial pressure, arterial dP/dt. and heart rate. These results with the low levels indicate that maneuvers which decrease venous return and central venous pressure in man can influence forearm vascular tone without significant changes in the determinants of carotid and aortic baroreceptor activity. During high levels of venous pooling (LBNP - 20 and - 40 mm Hg), significant decreases in arterial pressure and dP/dt and significant increases in heart rate accompanied the further reductions in central venous pressure, forearm blood flow, and forearm vascular conductance. About 73% of the decrease in conductance during venous pooling at LBNP - 40 mm Hg, which was sufficient to decrease arterial pressure and activate high pressure baroreceptor reflexes, occurred during low levels of venous pooling at LBNP - 10 mm Hg without changes in arterial pressure. This suggests that much of the forearm vasoconstriction with the high levels of venous pooling, which were sufficient to decrease arterial pressure, may be accounted for by reflexes originating in areas other than high pressure baroreceptors. The results of these studies suggest that low pressure baroreceptors exert an important influence on forearm vascular tone during decreases in venous return and central venous pressure in man.

Does vasopressin contribute to salt-induced hypertension in the Dahl strain?

A humoral factor has been implicated in Dahl salt-sensitive genetically hypertensive rats. The goal of this study was to evaluate the pressor role of vasopressin (AVP) in Dahl rats. Salt-sensitive (S) and resistant (R) rats were fed either high (8%) or low (.04%) NaCl diets for 6 to 8 weeks. Blood pressure was elevated in S rats fed high salt diets (p less than 0.05). Plasma AVP increased with high salt diet in both groups (p less than 0.05), but was higher in S than R rats (2.0 +/- 0.3 and 1.3 +/- 0.2 microU/ml respectively, mean +/- SE, p less than 0.05). With low salt diet, plasma AVP did not differ significantly in S and R rats (1.0 +/- 0.2 and 0.7 +/- 0.2 microU/ml respectively). Pressor responses to intravenous injection of AVP were greater in S than R rats (p less than 0.05), but this difference was also observed with pressor responses to norepinephrine (S greater than R, p less than 0.05); there was no difference in pressor responses to AVP in S rats fed high vs low salt diet. Injection of 50 micrograms of d(CH2)5 VDAVP, which selectively inhibits vasoconstrictor effects of AVP, failed to lower blood pressure in S and R rats fed high or low salt diets despite the fact that this dose decreased pressor responses to 8 microU of AVP more than 90%. Although plasma AVP and vasopressor responses to AVP and NE are slightly elevated in S rats fed high salt, results with d(CH2)5 VDAVP suggest that vasoconstrictor effects of AVP do not play an important role in the maintenance of hypertension in Dahl S rats.

Blood pressure and heart rate responses to microinjection of vasopressin into the nucleus tractus solitarius region of the rat.

The nucleus tractus solitarius (NTS) region in the rat has been shown to receive arginine vasopressin (AVP) and oxytocin (OT) neurophysin-containing neuronal projections from the suprachiasmatic (SNC) and paraventricular nucleus (PVN). Thus, vasopressin and oxytocin might have central influences on the circulation. This as investigated by measuring arterial blood pressure and heart rate (HR) responses following microinjection of vasopressin and oxytocin (0.1, 1.0 and 10.0 ng) into the right nucleus tractus solitarius region of rats anesthetized with urethane. Injections of vasopressin into nucleus tractus solitarius produced dose-related increases in blood pressure and heart rate. The effect of oxytocin on the blood pressure and heart rate was of a lesser magnitude without showing a dose-response relationship. Equivolumetric injections of vehicle and luteinizing hormone-releasing hormone (LH-RH) peptide had no detectable effect on blood pressure and minimal effect on heart rate. Injections of vasopressin into three different sites in the brain stem (1 mm anterior, posterior, and lateral to the tractus solitarius) did not produce significant hemodynamic changes. Intravenously injected vasopressin produced increments in blood pressure only at the highest dose level (10.0 ng) and a decrease rather than an increase in heart rate. Ganglionic blockade significantly reduced pressor responses to vasopressin injected into the nucleus tractus solitarius region and completely eliminated HR responses. Pretreatment of the nucleus tractus solitarius with a vasopressin antagonist abolished the blood pressure and heart rate responses produced by injection of vasopressin. These results suggest that vasopressin acts in the region of the nucleus tractus solitarius to exert a central action on the circulation.

The role of arginine vasopressin on peripheral cardiac parasympathetic nerve function in the rat.

In rats, arginine vasopressin augments bradycardia associated with baroreflex activation. We investigated whether modulation of peripheral cardiac parasympathetic nerve function by AVP may play a role in this effect. To accomplish this we utilized an in vivo model with which we previously demonstrated both adrenergic and peptidergic modulation of cardiac parasympathetic nerve function. Urethane-anesthetized rats (250-350 g) were prepared with arterial and venous catheters and ECG leads. The cervical vagi were sectioned, and propranolol (1 mg/kg, i.v.) was administered to eliminate reflex changes in heart rate. To investigate potential preganglionic modulation by AVP, the right vagus nerve was electrically stimulated (0.5 mA; 0.5 msec; 1-10 Hz). To observe postganglionic effects through nicotinic activation, carbachol (a mixed nicotinic and muscarinic agonist) was injected (0.5 to 4.0 micrograms/kg, i.v.). To observe direct cholinergic effects at the SA node, methacholine (a pure muscarinic agonist) was injected (0.5 to 4.0 micrograms/kg). All three trials were performed before (control) and during AVP infusion (20 micrograms.kg.min). No consistent, significant differences in vagal-, carbachol- or methacholine-induced bradycardia were observed between control and AVP groups. Since endogenous plasma levels of AVP in the control situation may have saturated any vasopressinergic effect prior to AVP infusion, the experiments were repeated in Brattleboro rats, genetically deficient in AVP. Again, no consistent differences in heart rate responses to parasympathetic activation were noted between control and AVP-infused groups. These results suggest that in rats, vasopressinergic augmentation of baroreflex-induced bradycardia is not mediated by an effect on the peripheral cardiac parasympathetic innervation. However, it remains to be investigated whether AVP-mediated sympathetic withdrawal disinhibits cardiac parasympathetic nerve function.

Presynaptic regulation of cardiac sympathetic function in hypoxic guinea pigs.

In the normal heart, presynaptic cholinergic muscarinic and alpha 2-adrenergic mechanisms modify the fractional rate constant for norepinephrine (NE) synthesis (kNE), an index of sympathetic neural function. To evaluate presynaptic regulation of kNE, conscious guinea pigs subjected to normoxia and then hypoxia (n = 7-8 in each group) were pretreated with 1) vehicle; 2) a cholinergic muscarinic antagonist, methyl atropine; 3) an alpha 2-antagonist, yohimbine; or 4) a combination of the two. An increase of kNE was determined from incorporation of radiolabeled tyrosine into NE in a control period (arterial PO2 130 +/- 1.7 Torr, PCO2 36 +/- 0.5 Torr) and during a hypoxic state (PO2 49.6 +/- 1.0 Torr, PCO2 36 +/- 0.5 Torr). Hypoxia activated kNE in the atrioventricular node and right ventricular moderator band in vehicle-treated animals (P less than 0.05). Sympathetic activation was more general, however, because alpha 2-presynaptic influence acted to limit kNE in all tissues tested (P less than 0.05) except muscle, spleen, and posterior left ventricle. Cholinergic muscarinic presynaptic restraint on kNE was detected during hypoxia only in the left atrial appendage and lung (P less than 0.05). These data indicate that hypoxia increases kNE in the heart, but restraint by cholinergic muscarinic and alpha 2-adrenergic presynaptic mechanisms limits increases in neurotransmitter synthesis and noradrenergic activation regionally.

Neurohypophyseal peptides in the developing rat fetus.

The quantitative changes in the content of the neurohypophyseal peptides, neurophysins, oxytocin, and vasopressin, were determined in the developing rat fetus. No neurohypophyseal peptides were found in 12-day fetuses. Neurophysins were first detected at day 13 and increased dramatically on day 14. The hormones vasopressin and oxytocin were not detected at day 13 and were measured at low levels at day 14. A 350-fold molar excess of neurophysin to hormones existed at day 14. From day 14 to day 19 the total content of neurophysin decreased while the content of vasopressin and oxytocin slowly increased. At day 19 there was a near molar equivalency between the content of neurophysin and that of the neurohypophyseal hormones. From day 18 to day 22 there was a sharp increase in the content of vasopressin while the content of neurophysin and oxytocin increased less dramatically. At term there was a molar excess of vasopressin, and the molar ratio of neurophysin to hormone at the time of delivery was 0.12. Measurement of vasopressin by different radioimmunoassays and by bioassay indicated no contribution of arginine-vasotocin to the measured vasopressin.

Norepinephrine release from guinea pig cardiac sympathetic nerves is insensitive to ryanodine under physiological conditions.

The activation of neurotransmitter release in nerve cells appears to be primarily dependent upon influx of extracellular Ca2+, most of which is thought to cross nerve terminal membranes through N-type Ca2+ channels. Events in skeletal and cardiac muscle, in contrast, are regulated to a greater extent by intracellular Ca2+ exchange between cytosol and intracellular organelles such as sarcoplasmic reticulum. It is not known to what extent corresponding intracellular organelles, i.e. endoplasmic reticulum (ER), contribute to cytosolic Ca2+ transients and norepinephrine (NE) release from cardiac sympathetic nerves. Heart rate and NE release were measured in isolated perfused guinea pig hearts during 1-min stimulations (5 V, 4 Hz, 2 ms) of the right stellate ganglia prior to (S1), during the administration of (S2), and after (S3) the removal of ryanodine (1 microM) from the perfusate. Ryanodine is a selective modulator of caffeine-sensitive Ca2+ stores in ER. Baseline heart rates decreased significantly in the presence of ryanodine, documenting its physiological effect on cardiac cells. However, there was no detectable effect of ryanodine on nerve-stimulated increase in heart rate or NE release. These results indicate that the ryanodine-sensitive intracellular Ca2+ stores do not play a major role in cardiac sympathetic neurotransmission.

Facilitation of baroreflex-induced bradycardia by stimulation of specific hypothalamic sites in the rat.

Hypothalamic stimulation generally inhibits baroreflex-induced bradycardia. However, we have noted discrete areas of the rat hypothalamus which facilitate reflex bradycardia. The effects of hypothalamic stimulation on baroreflex-induced changes in heart rate were investigated in urethane-anesthetized rats (1.2 g/kg, i.p.; n = 6) instrumented with femoral arterial and venous catheters. Bipolar electrodes (250 micron diameter) were implanted stereotaxically in the hypothalamus. Baroreflex-induced bradycardia was elicited by phenylephrine (PE) injection (8-20 micrograms/kg). Responses to stimulation (STIM) (50-150 microA, 80 Hz, 0.5 ms), PE, and Stim + PE were studied for 1 min. In the ventral medial and anterior hypothalamus, STIM caused transient increases in blood pressure and no changes in heart rate. Peak blood pressure was lower during STIM + PE than during PE (144 +/- 5 vs 164 +/- 3 mm Hg; P less than 0.05). However, STIM + PE resulted in a lower heart rate compared to PE (194 +/- 22 22 vs 270 +/- 17 bpm; P less than 0.05). At 1 min, the heart rate in STIM + PE rats remained lower than in PE rats (205 +/- 37 vs 319 +/- 16 bpm; P less than 0.05). Atropine administration indicated that the facilitation was primarily parasympathetic in nature. These results identify specific hypothalamic regions which facilitate baroreflex-induced bradycardia by parasympathetic mechanisms.

Increased norepinephrine turnover in the median preoptic nucleus following reduced extracellular fluid volume.

The role of noradrenergic input to fluid balance regulatory systems in the anterior hypothalamus was studied by examination of norepinephrine (NE) turnover during reduction of systemic extracellular fluid volume. Extracellular fluid volume was decreased iso-osmotically by subcutaneous polyethylene glycol (PEG), known to increase thirst and vasopressin secretion. NE turnover was assessed by measuring the decline of NE concentration in brain micropunches after administration of alpha-methyl tyrosine in PEG- or sham-treated groups. Several hypothalamic areas were investigated, including the median preoptic area (MnPO), preoptic area (POA), paraventricular nucleus, supraoptic nucleus (SON), subfornical organ, ventromedial hypothalamus, and posterior hypothalamus. Volume-depleted animals showed significantly increased NE turnover in the MnPO, an important area for integration of fluid balance information. The POA and the SON also showed trends toward increased NE turnover. All other areas showed no difference in NE turnover between volume-depleted and normal animals. These results are consistent with previous findings that NE innervation to the MnPO is important in the control of fluid balance and also support the hypothesis that basal forebrain NE projections facilitate thirst and vasopressin secretion.