The influence of (+)-propranolol on the inotropic effect of dihydro-ouabain in relation to stimulation frequency

Effects of bucumolol, nadolol and nifenalol on maximum upstroke velocity of action potential in guinea pig papillary muscles

The effects of bucumolol (BUC), nadolol (NAD) and nifenalol (NIF) on contractile forces and on action potentials (APs) were investigated in isolated guinea pig atrial and papillary muscles, respectively. Log 1/ED40 values for the negative inotropic effects of these drugs were 0.097, 10 and 0.74 mmol/l in this order. BUC (50 mumol/l), NAD (0.5 mmol/l) and NIF (0.2 mmol/l) produced about 60, 20 and 20% reduction of Vmax at 1 Hz. The frequency-dependent reductions at these and higher concentrations were greatest for BUC, intermediate for NAD and least for NIF. These potencies at certain frequencies were, as a whole, consistent with log P-potency relationship established in our previous papers (Harada et al. 1981; Ban et al. 1985). The reductions of Vmax in APs in response to premature stimuli during basic stimuli at the rate of 0.25 or 0.027 Hz decayed exponentially during diastolic intervals (DI). The time constants of these decay process (tau) estimated by linear and nonlinear regression analyses and by eye were 12.2-9.6 s for BUC (50-100 mumol/l) and 2.9-4.8 s for NAD (1-2 mmol/l) and 57-87 ms for NIF (0.2-1 mmol/l). In terms of the molecular weight (MW)-log tau relationship (Ban et al. 1985), these tau values are within the 95% fiducial limit for BUC and NAD and deviated from the lower fiducial limit for NIF. The frequency-dependent reductions of Vmax by these drugs were explained in terms of a function of tau and the intercept Ao. Based on the study made by Cohen et al.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of atenolol and nifedipine on digoxin-induced inotropism in humans

Short term effect of digoxin on left ventricular performance was studied in six healthy volunteers before and during atenolol or nifedipine administration. Left ventricular function was evaluated by systolic time intervals and echocardiography. No changes in left ventricular end diastolic or systolic dimensions occurred throughout the study, indicating unchanged ventricular pre- and afterload. Thus, changes in the systolic time intervals must be attributed to changes in cardiac contractility. Changes in the pre-ejection period index (PEPI) obtained from the systolic time intervals were used as a measure of digoxin-induced inotropism. A concentration-response relationship between plasma digoxin level and changes in PEPI was revealed when digoxin was given alone. Atenolol did not influence the digoxin-induced inotropism at a given serum digoxin level. During nifedipine administration no inotropic effect of digoxin could be demonstrated. Thus, it is concluded that nifedipine attenuates digoxin-induced inotropism, while atenolol seems without this effect. These results are in accordance with previous experiments and reflect the different pharmacological sites of action of beta-adrenoceptor antagonists and calcium channel blocking agents. Plasma digoxin concentration, renal digoxin clearance and creatinine clearance did not change during atenolol or nifedipine.

Enhancement of calcium current during digitalis inotropy in mammalian heart: positive feed-back regulation by intracellular calcium?

1. Effects of digitalis compounds on slow inward Ca current I(si)) and contractile force were examined in ferret ventricular muscle (single sucrose-gap voltage clamp) and calf Purkinje fibres (two micro-electrode voltage clamp).2. In ventricular muscle, ouabain increased I(si) and inward current tails associated with I(si) conductance. The enhancement of I(si) followed a time course similar to the development of the positive inotropic effect, and it could be observed in the absence of aftercontractions or other signs of toxicit.3. The response of myocardial I(si) and twitch force to ouabain depended strongly on a previous history of driven action potentials.4. Veratridine, a toxin that promotes Na entry through tetrodotoxin-sensitive channels, also increased I(si) and twitch force in driven ventricular muscle preparations.5. The effects of ouabain, action potential stimulation and veratridine are consistent with reported effects of K-poor solutions in indicating that elevation of intracellular Na can lead to enhancement of I(si). Additional experiments suggest that the link between Na(i) and I(si) involves intracellular Ca.6. When Cs-loaded Purkinje fibres were bathed in solutions containing Sr instead of Ca, enhancement of I(si) by strophanthidin was abolished even though a positive inotropic response persisted.7. After intracellular injection of Purkinje fibres with EGTA, I(si) no longer increased with strophanthidin, although it remained responsive to adrenaline.8. Clear-cut increases in I(si) were seen in Cs-loaded Purkinje fibres even at very low concentrations of strophanthidin (20-50 nM), where the occurence of Na pump inhibition has been questioned.9. Positive regulation of Ca entry by intracellular Ca may act as a facilitory mechanism that amplifies myocardial responsiveness to digitalis and other inotropic interventions. Through changes in I(si), small rises in diastolic free Ca might lead to large increases in the activator Ca transient during contraction.

Stimulant and depressant effects of beta-adrenoceptor blocking agents on isolated heart muscle. A positive inotropic effect not mediated through andrenoceptors

The chronotropic and inotropic effects of fourteen beta-adrenoceptor blocking agents were studied in vitro on preparations of isolated heart muscle from kittens and guinea pigs. Most of the agents exert negative inotropic and chronotropic effects that increase rapidly with concentration above 10(-6) M. Exceptions are the closely related compounds practolol and atenolol, which have minimal depressant effects in concentrations as high as 2 X 10(-3) M. Dose-response relations for the depressant effects are similar for pacemaker frequency and for tension development in atrial and papillary muscle. The cardiodepressant effects of beta-blockers are non-stereospecific and are apparently unrelated to the actions of the drugs at beta-adrenoceptors. Many beta-blockers exert positive inotropic and chronotropic effects at concentrations lower than those that depress. The stimulant effects are slow in onset and do not fade. In most instances these effects are blocked by propranolol and may therefore be considered to be mediated throught beta-adrenoceptors; (-)-enantiomers are more potent than (+)-enantiomers as adrenoceptor stimulants. Adrenoceptor-mediated inotropic effects are usually more pronounced in atrial than in ventricular muscle. Certain beta-blockers, notably INPEA and sotalol, exert positive inotropic effects that are not blocked by propranolol or phenyoxybenzamine. The effects are very slow in onset and offset, and are accompanied by the development of contractions with a late tonic component which coincides with a greatly prolonged action potential. Unlike the adrenoceptor-mediated effect, the non-sympathomimetic inotropic effect is more pronounced in ventricular than in atrial muscle, and is not stereospecific. Pindolol also causes the development of a late tonic component, but the accompanying positive inotropic effect is overcome by the simultaneous development of the depressant action of the drug.