Relaxant effects of amrinone upon pulmonary smooth muscle

Comparative effect of amrinone, aminophylline and diltiazem on rat airway smooth muscle

We investigated the effect of amrinone, a phosphodiesterase III inhibitor, on rat airway smooth muscle, and thereafter, compared its activity with aminophylline and diltiazem. Amrinone produced relaxation of the acetylcholine-induced airway contraction in a dose-related manner. This bronchodilatory activity of amrinone was similar to that of aminophylline, but smaller than that of diltiazem. The 50% relaxant effect (ED50) of amrinone, aminophylline and diltiazem were 3.6 x 10(-4) M, 1.4 x 10(-4) M and 1.4 x 10(-5) M, respectively. Diltiazem was the most potent airway relaxant, and amrinone was less potent in these experiments. Taken together in its positive inotropic and chronotropic effects and anti-inflammatory activity, however, amrinone could be beneficial for treatment of patients suffering from asthma or heart failure with cardiac asthma.

Effects of amrinone on the electromechanical coupling in frog skeletal muscle fibres

The contractile effects of 1.1 mM amrinone were studied on isolated skeletal muscle fibres of the frog (2-5 degrees C, slack length). Amrinone potentiated the twitch amplitude and the maximum tetanic tension and also enhanced the maximum contracture response elicited by 117.5 mM K+ (mean increase 6.3 +/- 1.8%, n = 6, P less than 0.02). The time to half-relaxation of the potassium contracture was slightly increased (mean change 13.8 +/- 3.5%, n = 7, P less than 0.01). Amrinone furthermore shifted the S-shaped curve relating contracture tension to log K+ to the left, and thus reduced the threshold depolarization (the mechanical threshold) needed to elicit a contracture (mean reduction 11 +/- 1 mV). The duration of the action potential at the -25 mV level was slightly increased by amrinone, whereas the resting membrane potential was unaffected.

Effects of amrinone on twitch, tetanus and shortening kinetics in mammalian skeletal muscle

The contractile effects of amrinone were studied on twitch and tetanus responses of mouse lumbrical muscles. Amrinone (1.1 mM) increased the maximum force level during the tetanus, reduced the rate of rise of force during the onset of tetanus and prolonged the time from the last stimulus to half-relaxation. The rate of redevelopment of force after a release during the tetanus plateau was likewise reduced. Amrinone (greater than or equal to 0.5 mM) increased the time to peak twitch force and the time to half-relaxation during the twitch. The peak twitch force was not significantly changed. The force-velocity relation was markedly affected by amrinone: there was a decrease in maximum velocity of shortening, an increase in maximum isometric force and a decrease in curvature of the force-velocity relation. The results suggest that amrinone modulates the kinetic properties of the myosin cross-bridges by exerting a specific effect upon the myofilament system. The latter effects are mainly responsible for the contractile changes produced by amrinone in mouse skeletal muscle. Amrinone appears to have little effect on the metabolism of activator calcium in this particular preparation.

Effects of amrinone on the contractile behaviour of frog striated muscle fibres

The effects of amrinone (0.005-2.1 mM) were studied on isolated single muscle fibres of the frog (0.6-4.3 degrees C). In the concentrations used amrinone potentiated the isometric twitch, increased the initial rate of force development and prolonged the time to peak twitch force and the time from peak force to half relaxation. There was a moderate but statistically significant (P less than 0.001) enhancement of the maximum tetanic force and a marked slowing of the relaxation phase of the tetanus. Amrinone depressed the velocity of unloaded shortening by 33 +/- 2% (mean +/- SE, n = 6) and reduced the curvature of the force-velocity relation leading to an increase of a/Po in Hill's (1938) hyperbolic equation. In accordance with the effects of amrinone on the force-velocity relation, there was a decrease in the rate of redevelopment of force after the fibre was released to zero tension during the tetanus plateau. The evidence suggests that the drug affects the contractile properties of frog muscle fibres by two different mechanisms: (I) amrinone affects the excitation-contraction coupling causing enhanced calcium release in response to membrane excitation and (2) amrinone exerts a direct effect on the kinetics of turnover of the myosin cross-bridges. The latter aspect of the drug effect was evaluated in terms of A.F. Huxley's (1957) cross-bridge model by means of a computer program.