Differential inhibition of alpha adrenoceptor-mediated pressor responses by rat atrial natriuretic peptide in the pithed rat

Previous investigations have shown that rat atrial natriuretic peptide (r-ANP,5-28, atriopeptin III), antagonizes the effects of various pressor hormones (angiotensin II, vasopressin and norepinephrine) but is ineffective against pressor responses to acute spinal cord stimulation. Because the latter are believed to be mediated by intrajunctional alpha-1 adrenoceptors, whereas the others are thought to involve mainly extrajunctional receptors, we explored the possible specificity of r-ANP for alpha adrenoceptor subtypes, by comparing r-ANP, the calcium channel blocker nifedipine and the vasodilator sodium nitroprusside in their ability to inhibit pressor responses to the alpha-2 and alpha-1 adrenoceptor agonists, clonidine and phenylephrine, in pithed, vagotomized rats. Acute pressor responses to bolus-injected clonidine were dose-dependently attenuated by both r-ANP (up to 21%) and nifedipine (up to 37%), but acute pressor responses to phenylephrine were unaffected. Sodium nitroprusside inhibited pressor responses to clonidine (up to 67%) and phenylephrine equally (up to 66%). Pressor responses during constant infusions of clonidine and phenylephrine were attenuated similarly by r-ANP and nifedipine. This pattern of results, alpha-2 adrenoceptor specificity during immediate pressor responses but not during sustained pressor responses, suggests that r-ANP, like nifedipine, attenuates those adrenoceptor-mediated pressor responses which depend on slow transmembrane calcium fluxes.

Arrhythmogenic dose of acetylstrophanthidin unchanged by beta-sympathomimetics in conscious dogs

The enhanced arrhythmogenic risk of combined treatment with cardiac glycosides and beta-sympathomimetics is referred in some textbooks, but only a few detailed studies on in vivo models are available. We therefore investigated this problem in conscious dogs in an intraindividual study. We determined the dose of acetylstrophanthidin (intravenous infusion of 5 mcg/kg per min), which provoked ventricular premature beats with and without concomitant treatment with the partial beta-agonistic compounds doxaminol (3 mg/kg p.o.), prenalterol (0.4 or 1.0 mg/kg p.o.) or isoprenaline (0.31 +/- 0.100 mcg/kg per min). In some dogs a coronary artery was narrowed in order to reduce the coronary blood supply. The arrhythmogenic dose of acetylstrophanthidin was nearly the same in all the groups investigated (range from 52.1 +/- 5.66 to 59.9 +/- 3.23 mcg/kg). Whereas the arrhythmogenic dose of acetylstrophanthidin was unchanged by beta-sympathomimetics, the combination of the glycoside and each of the beta-agonistic drugs increased the contractile force more than did either single compound. We therefore conclude that the arrhythmogenic risk of the combination of glycosides and beta-sympathomimetics may be--at least in experimental models--less than has been suggested in the past.