Efficacy and safety of the novel Na+,K(+)-ATPase inhibitor 20R 14 beta-amino 3 beta-rhamnosyl 5 beta-pregnan 20 beta-ol in a dog model of heart failure.

20R 14 beta-amino 3 beta-rhamnosyl 5 beta-pregnan 20 beta-ol (LND 623, CAS 90520-42-6) was investigated and compared to digoxin in anesthetized dogs. The hemodynamic profiles showed: a) a pure positive inotropic action of LND-623; b) its potency was four-fold higher than that of digoxin and more marked in heart failure; c) its duration of action was maintained for at least 6 h. The onset and reversal of the inotropic effects of a single dose (3.3 nmol.kg-1.min-1) were faster with LND-623 than those of digoxin. This reversal is consistent with the faster dissociation profile observed at the level of the high affinity cardiac Na+,K(+)-ATPase receptor form. The advantage of LND-623 over digoxin resides in its larger therapeutic index (ratio of arrhythmogenic to inotropic responses) in anesthetized dogs with propranolol-induced heart failure. This index was 6 for LND-623 and 2 for digoxin.

Tissue localization of active Na,K-ATPase isoenzymes by determination of their profile of inhibition with ouabain, digoxin, digitoxigenin and LND 796, a new aminosteroid cardiotonic.

Recently, Na,K-ATPase isoforms with differential affinities for digitalis have been identified that may contribute to different toxicity profiles. Our objectives were to localize them and to define tissue receptor patterns by examining the effect of different glycosides on the Na,K-ATPase activity. The digitalis derivatives used exhibit variation in lipophilicity and rate of enzyme inhibition. Membrane fractions enriched in Na,K-ATPase were prepared from canine heart, brain, aorta and peripheral nerves. The inhibition of enzyme activities indicates a pattern of differential sensitivities with IC50 values starting from 3 nM in heart and 30 nM in brain. Therefore, high and low affinity active forms of the Na,K-ATPase enzyme coexist in these tissues. The data also suggest the existence of two Na,K-ATPase isoforms in aorta and peripheral nerves as identified by the action of digitoxigenin and LND 796 where the predominant expression is that of a high affinity form. The comparison of the patterns of digitalis sensitivities in these different tissues, suggests a more complex molecular interaction than that which can be explained by the presence of only two forms.

A new inotropic aminosteroid: LND 623.

It is usually postulated that the side chain attached to 17 beta position on the steroid nucleus of the cardioactive glycosides is a major determinant of their pharmacological activity. A new aminosteroid (LND 623) has been prepared. Despite the fact that the structural features quoted above are lacking, the product demonstrates a strong inotropic effect. LND 623 was active at the same range of concentrations as ouabain or digoxin. However, the maximum increase of the contractile force was significantly higher. The strength of the inotropic action is not modified in the presence of propranolol. The preliminary results suggest that the molecular requirements for the interaction of the drugs with the 'inotropic receptor' may be re-examined.

Studies on the mechanism of the vasodilator effects of prazosin in dogs and rabbits.

In pentobarbital (35.0 mg/kg) anaesthetised dogs, bolus injections of prazosin into the femoral artery (3.0--300.0 microgram) provoked a dose-related fall in the vascular resistance of the innervated hind limb. In contrast to papaverine, prazosin failed to produce the same effect in dogs under spinal anaesthesia even when the intrinsic femoral vascular tone was increased with vasopressin. However, vasodilator effects of prazosin were again observed when the tone of the limb was elevated by either stimulating the sympathetic lumbar chain or by infusing alpha-adrenoceptor agonists. A significant reduction of both aortic blood pressure and pressor response to bilateral carotid artery occlusion was noted in a group of normotensive dogs anaesthetised 12 h after the last dose of prazosin given twice daily at 0.5 mg/kg, p.o., for 3 day period. This short-term treatment modified neither the resting heart rate nor the positive chronotropic effect induced by either intravenous noradrenaline or electrical stimulation of pre- and post-ganglionic nerve fibres of the right stellate ganglion. However, it prevented the larger increase in heart rate in response to bilateral carotid occlusion in placebo-treated dogs after section of the vagi. A decrease in baseline sympathetic tone of the perfused hind limb as well as vasoconstrictor effects produced by i.a. injections of several alpha-adrenoceptor agonists and electrical stimulation of the lumbar sympathetic chain was observed in prazosin-treated animals. The dose--pressor response profiles to these alpha-adrenoceptor stimulants after prazosin were not parallel to those obtained in the control group. The vasoconstrictor response to angiotensin II was not changed by prazosin. In rabbit aortic strips, prazosin (0.1--3.0 micrometer) produced competitive antagonism of the contractile responses induced by cirazoline, noradrenaline and phenylephrine. In contrast to papaverine, prazosin in concentrations up to 100.0 micrometer neither relaxed the aortic strips contracted by potassium ions nor modified the concentration-response curve to calcium ions. These studies indicate that blood pressure lowering effects of prazosin given acutely or for three days can be accounted for by a clear-cut functional impairment of vascular postsynaptic alpha-adrenoceptors. No evidence for a direct myorelaxant property of prazosin could be obtained in these studies.

Vascular beta-adrenoceptor stimulating properties of phenylephrine.

In pithed rats pretreated with propranolol changes occurred in the slopes of dose--pressor response curves to i.v. phenylephrine and noradrenaline but not to LD 3098, a pure alpha-adrenoceptor stimulant. In dogs, cats, and spontaneously hypertensive rats pretreated with an alpha-adrenoceptor blocker, i.v. phenylephrine induced vasodilatory responses which were antagonised by propranolol. These results indicate that phenylephrine, in addition to its known beta1-agonistic properties, stimulates the vascular beta2-adrenoceptors.