Robert L. Wendt

Regional vasodilator actions of calcitonin gene-related peptide in conscious SHR

In the present study the regional hemodynamic effects of CGRP were examined in conscious unrestrained spontaneously hypertensive rats (SHR). The animals were chronically instrumented with miniaturized pulsed Doppler flow probes to allow continuous measurement of renal, mesenteric and hindquarter blood flow. Bolus intravenous injection of CGRP (0.1-5 micrograms/kg) produced a dose-dependent fall in mean arterial pressure (maximal change = -48 +/- 5 mmHg) which was accompanied by a marked tachycardia (maximal change = 143 +/- 16 b/min). Depressor responses to CGRP were sustained for approximately 3-5 min. CGRP markedly reduced regional vascular resistance in all three vascular beds. No regional-selective vasodilator response was observed. These data indicate that CGRP is a potent vasodilator peptide in conscious SHR. The study suggests further that CGRP may contribute to the physiologic regulation of cardiovascular function.

Effects of atrial natriuretic factor on drinking responses to central angiotensin ii

The present study examined the effects of ANF(4-28) [Wy-47,663], a synthetic 25 amino acid human atrial natriuretic factor, on the dipsogenic actions of centrally-administered angiotensin II in conscious rats. Bolus injection (100 ng) or continuous infusion (60 ng/min) of Wy-47,663 or vehicle into the lateral cerebroventricle had no effect on mean arterial pressure or heart rate. No obvious behavioral changes were observed after central administration of Wy-47,663 or vehicle. Central injection of angiotensin II (15 or 30 ng) promptly elicited prolonged drinking responses in vehicle-treated rats. In rats pretreated with Wy-47,663, the onset of the angiotensin II-induced drinking responses was significantly delayed compared to vehicle-treated animals. However, Wy-47,663 had no effect on the total volume consumed over 30 minutes after angiotensin II injection. Intravenous infusion of Wy-47,663 (2 micrograms/kg/min) failed to alter the dipsogenic action of centrally administered angiotensin II. These data indicate that atrial natriuretic factor found within the brain but not the peripheral circulation may participate in the regulation of extracellular fluid volume by modulating the dipsogenic actions of the central renin-angiotensin system.

Hemodynamic effects of calcitonin gene-related peptide in conscious rats

The cardiovascular effects of calcitonin gene-related peptide (CGRP) were examined in conscious, unrestrained rats. Changes in mean arterial pressure, heart rate and cardiac output were continuously monitored before and after i.v. bolus injection of CGRP (0.1-5 micrograms/kg). Injection of the peptide caused dose-dependent reductions in mean arterial pressure (-24 +/- 4 mmHg), which were accompanied by marked tachycardia. Cardiac output was significantly increased after CGRP but little change was observed in stroke volume. CGRP also reduced total peripheral resistance (-46 +/- 6%). These data indicate that the hypotensive actions of CGRP are mediated through peripheral vasodilation rather than through reductions in cardiac output. Pretreatment with propranolol significantly reduced the tachycardia responses to CGRP from 81 +/- 11 beats/min to 36 +/- 4 beats/min, but did not abolish the increase in heart rate. These data suggest that CGRP produces a tachycardia through reflex increases in cardiac sympathetic tone and through possible direct positive chronotropic effects on the heart.

Structural variants of verapamil and W-7 with combined Ca2+ entry blockade/myosin phosphorylation inhibitory mechanisms.

Arylalkylsulfonamides, identified as Wy-46,622 and Wy-47,324, combine pharmacological properties of the Ca2+ entry blocker verapamil and the calmodulin antagonist W-7. These agents directly inhibit arterial actin-myosin interactions via inhibition of myosin light chain (MLC) phosphorylation in actomyosin. Potencies of both Wy-46,622 (IC50 = 26 μM) and Wy-47,324 (IC50 = 18 μM) are greater than W-7 (IC50 = 35 μM); verapamil is inactive at 100 μM. Both Wy-46,622 and Wy-47,324 are less potent than verapamil, but more potent than W-7, at inhibiting K+ -depolarized force development in paced rabbit atria. At higher concentrations (30 μM) which inhibit MLC phosphorylation, Wy-46,622 and Wy-47,324 are either equal to or more efficacious than verapamil in inhibiting receptor-mediated contractions in intact porcine coronary (histamine. serotonin, prostaglandin F2a, carbocyclic thromboxane A2) or guinea pig aortic (leukotriene C4) smooth muscle. Both Wy-47,324 (IC50 = 16 μM) and Wy-46,622 (IC50 = 23 μM) are inhibitors of the second phase of epinephrine-induced human platelet aggregation; Wy-47,324 is more potent than verapamil or W-7 (IC50 for each = 29 μM). These results suggest that these agents possess combined Ca2+ entry blocker/MLC phosphorylation inhibitory mechanisms. Furthermore, they are more vascular specific than verapamil, and are effective inhibitors of intra-cellular Ca2+ -mediated events in vascular smooth muscle and platelets.

Chapter 26. Atrial Natriuretic Factor

Publisher Summary The heart influences renal functions through the release of humoral substances. Dense core granules, resembling secretory granules, were observed in the atria of mammalian hearts. The granules were in close association with the golgi complex and contained a protein material. DeBold and co-workers demonstrated that intravenous injection of atrial, but not ventricular, extracts from rat hearts into anesthetized rats caused a marked but short-lived natriuresis and diuresis and lowered arterial pressure. The unknown material was referred to as atrial natriuretic factor (ANF). The substance was isolated, purified and sequenced; the DNA sequence of the precursor peptide was discovered and receptor sites were identified; the pharmacologic actions of the natriuretic peptide were examined; assays for measurement of ANF were devised, and the clinical effects of ANF were reported. This chapter presents a brief summary of the biology of ANF. The chapter discusses peptide sequence and gene isolation, receptor localization and second messengers, renal pharmacology, and hemodynamic and endocrine effects of ANF. The pharmacology of ANF indicates that it eventually may lead to the discovery of effective agents for the treatment of edema in patients with congestive heart failure, hypertension, renal insufficiency or hepatic cirrhosis. Unlike current diuretic therapy, ANF may prevent compensatory increases in renin release and may be a potassium-sparing diuretic, through direct antagonism of aldosterone release. ANF possesses advantages over many vasodilator agents, in that reflex tachycardia and urinary salt retention are not generally associated with the depressor actions of the atrial peptides. Indeed, initial clinical trials with ANF indicate that the atrial peptides are quite effective in humans.

Wy-46,300 and Wy-46,531: Vascular Smooth Muscle Relaxant/antihypertensive Agents with Combined Ca2+ Antagonist/myosin Phosphorylation Inhibitory Mechanisms

1,4,5,6,7,8-hexahydro-2-methyl-5-oxo-4-(pentafluorophenyl) −1,7 - naphthyridine - 3 - carboxylic acid methyl ester hydrochlorides with either a 3-phenoxy-2-hydroxypropyl (Wy-46,300) or 2-hydroxy-4-phenylbutyl (Wy-46,531) substituent at the 7 position are structurally novel vascular relaxant compounds which lower blood pressure by ∼35 mm Hg at 25 mg/kg p.o. in spontaneously hypertensive rats (SHR) or perinephritic hypertensive beagles. In contrast to standard Ca2+ blockers, both agents also directly inhibit myosin phosphorylation and actin-myosin interactions in Triton-purified arterial actomyosin. The potency of Wy-46,300 (IC50 = 28 μM) in this system is similar to the calmodulin antagonist W-7, whereas Wy-46,531 (IC5 = 18 μ M) is more potent. Inhibition of both parameters is attenuated in the presence of maximal Ca2+-calmodulin, consistent with calmodulin antagonism as the mechanism for myosin phosphorylation inhibition. In intact smooth muscle, these agents inhibit K+-depolarized contractions of rabbit aortic strips in a biphasic manner (0.1–10 μ M, 30–40% inhibition; 10 μ M/-100 μ M, 90% inhibition). In contrast to comparable inhibition of force by standard Ca2+ blockers, this latter phase of inhibition is not reversible with the Ca2+ agonist Bay K 8644. Moreover, significantly greater inhibition of receptor-mediated contractions (norepinephrine, angiotensin II, histamine) than that observed with nifedipine is apparent with Wy-46,300. In paced rabbit atria, both Wy-46,300 and Wy-46,531 are 10–100 times less potent as negative inotropic agents (IC25 = 3–5 μ M) when compared with standard Ca2+ blockers. These results suggest that Wy-46,300 and Wy-46,531 are vascular relaxants which appear to act through combined inhibition of Ca2 + entry and direct inhibition of Ca2+ calmodulin-dependent activation of myosin phosphorylation.

Effects of nifedipine on the hypotensive actions of alpha 2-agonists in conscious spontaneously hypertensive rats.

The present study was designed to examine the effects of the calcium entry blocker, nifedipine, on the acute depressor responses to alpha 2-adrenoceptor agonists. Conscious spontaneously hypertensive rats (SHR) were given nifedipine intravenously (25 micrograms/kg) or vehicle (polyethylene glycol). Subsequent intravenous administration of guanabenz (10, 25, or 50 micrograms/kg) or clonidine (5 micrograms/kg) caused biphasic changes in mean arterial pressure (MAP) in vehicle-treated SHR, an initial pressor response, which was followed by a sustained fall in MAP. In nifedipine-treated rats, the pressor actions of guanabenz and clonidine were virtually abolished, while the onset of the depressor responses were significantly accelerated. The maximal depressor responses to guanabenz and clonidine were similar in vehicle- and nifedipine-pretreated SHR. Central administration of nifedipine (5, 10, or 20 micrograms/kg) in the lateral cerebroventricle significantly attenuated the depressor responses to guanabenz in a dose-dependent manner as compared to vehicle-treated rats. Oral pretreatment with nifedipine (2.5 mg/kg) also significantly blunted the hypotensive actions of guanabenz in the conscious SHR. These data indicate that central and oral administration of nifedipine antagonized the centrally mediated hypotensive actions of guanabenz. Intravenous bolus administration of nifedipine failed to attenuate the maximal depressor responses to guanabenz, suggesting that insufficient nifedipine accumulated centrally to affect central alpha 2-adrenoceptor function.