Influence of papaverine and isoprenaline on contractility and cyclic AMP level of left guinea-pig atria at different rates of beat

The role of cyclic AMP in the positive inotropic effect mediated by beta 1- and beta 2-adrenoceptors in isolated human right atrium

The time course of the effects of isoprenaline (3 X 10(-7) mol/l) on contractile force and on the cyclic AMP level was studied in the electrically driven isolated muscle strip of the human right atrium. Isoprenaline produced a rise in cyclic AMP content (maximum increase after 60 s) preceding the increase in contractile force. The effects of isoprenaline on contractile force and on the intracellular level of cyclic AMP were enhanced in the presence of the phosphodiesterase inhibitor papaverine (10(-5) mol/l). On the other hand, the beta-adrenoceptor antagonist propranolol (10(-7) mol/l) suppressed isoprenaline-induced cyclic AMP increases, but reduced the increase in force of contraction by only 35%. In addition, both the beta 1-selective antagonist bisoprolol (3 X 10(-9)-3 X 10(-8) mol/l) and the beta 2-selective antagonist ICI 118,551 (3 X 10(-9)-3 X 10(-8) mol/l) inhibited the isoprenaline-induced cyclic AMP increase concentration-dependently; ICI 118,551 produced more pronounced inhibition than bisoprolol. It is concluded that cyclic AMP is involved in the positive inotropic action of isoprenaline evoked by beta 1- and beta 2-adrenoceptor stimulation in isolated human right atrium; however, an additional cyclic AMP independent mechanism cannot be ruled out.

Papaverine-induced positive inotropism with failure to increase cyclic AMP in rat atria

Phosphodiesterase inhibition by papaverine is likely to play a minor role in the rat atrial inotropic response because non-significant changes in cyclic AMP were obtained. Isoprenaline however raised the nucleotide levels three-fold and up to seventeen-fold when papaverine was added in a similar preparation. A prominent effect of papaverine was to lengthen relaxation instead of shortening it as did isoprenaline. The results suggest different sites of action, although overlapping effects cannot be excluded.

Effects of papaverine on calcium efflux and contractility in superfused rat left atria

The effects of papaverine upon force of contraction, maximal rate of contraction, maximal rate of relaxation and 45Ca efflux were studied in isolated superfused rat left atria electrically driven at 1 Hz. Papaverine (3 X 10(-5) mol/l, increased developed tension (from 5.35 +/- 1.17 mN to 7.18 +/- 1.51 mN) by 1.8 +/- 0.41 mN (+33%, p less than 0.01) and maximal rate of contraction (+T) by 34.5 +/- 13% (p less than 0.05). In all experimental conditions tested, papaverine increased the rate of 45Ca efflux. The amount by which papaverine increased 45Ca efflux was 175 +/- 41 nmoles . g wet wt-1 and 304 +/- 76 nmoles . g wet wt-1, in the presence and in the absence of caffeine, respectively. The plot of changes in 45Ca efflux versus changes in developed tension fitted to a straight line (r = +0.56, n = 17, p less than 0.05), with a slope and intercept of 42 nmoles Ca . mN-1 . g wet wt-1 and -0.47 mN respectively, suggesting an association between the changes induced by papaverine in force development and increased 45Ca efflux.

Dissociation of cyclic AMP and contractile responses to isoprenaline: effects of a dihydropyridine derivative, nicardipine (YC-93), on canine ventricular muscle

Nicardipine (YC-93), a 1,4-dihydropyridine derivative, inhibited the cyclic AMP phosphodiesterase (PDE) activity of purified PDE in a cell-free preparation. Its inhibitory action on the purified PDE was about seven times that of papaverine. On the other hand, YC-93 did not affect the intracellular cyclic AMP level and the accumulation of cyclic AMP caused by isoprenaline in the isolated canine right ventricular myocardium. YC-93 caused a prominent negative inotropic action which developed gradually to reach a steady level 1 h after its administration. The potency of YC-93 to depress the force of contraction was one tenth that of D600. The dose-response curve for isoprenaline was shifted to the right and downward in the presence of YC-93 in a concentration-dependent manner, and the positive inotropic action of calcium was also inhibited markedly by YC-93. The depressant action of YC-93 on the positive inotropic actions of isoprenaline and calcium was more prominent than that of D600. Although YC-93 is a potent PDE inhibitor in the cell-free preparation, the PDE in the intact cell system may be not accessible to the drug. Thus, it is considered that YC-93 acted as a calcium antagonistic drug on the isolated canine ventricular myocardium, and thereby inhibited the positive inotropic action of isoprenaline without affecting the intracellular accumulation of cyclic AMP caused by isoprenaline.