The inotropic mechanism of the phosphodiesterase inhibitor OPC 3911 alone and in combination with rolipram, studied in papillary muscles of ferret and guinea-pig and isolated myocytes of guinea-pig ventricular muscle

Modeling short-term interval-force relations in cardiac muscle

This study employs two modeling approaches to investigate short-term interval-force relations. The first approach is to develop a low-order, discrete-time model of excitation-contraction coupling to determine which parameter combinations produce the degree of postextrasystolic potentiation seen experimentally. Potentiation is found to increase 1) for low recirculation fraction, 2) for high releasable fraction, i.e., the maximum fraction of Ca(2+) released from the sarcoplasmic reticulum (SR) given full restitution, and 3) for strong negative feedback of the SR release on sarcolemmal Ca(2+) influx. The second modeling approach is to develop a more detailed single ventricular cell model that simulates action potentials, Ca(2+)-handling mechanisms, and isometric force generation by the myofilaments. A slow transition from the adapted state of the ryanodine receptor produces a gradual recovery of the SR release and restitution behavior. For potentiation, a small extrasystolic release leaves more Ca(2+) in the SR but also increases the SR loading by two mechanisms: 1) less Ca(2+)-induced inactivation of L-type channels and 2) reduction of action potential height by residual activation of the time-dependent delayed rectifier K(+) current, which increases Ca(2+) influx. The cooperativity of the myofilaments amplifies the relatively small changes in the Ca(2+) transient amplitude to produce larger changes in isometric force. These findings suggest that short-term interval-force relations result mainly from the interplay of the ryanodine receptor adaptation and the SR Ca(2+) loading, with additional contributions from membrane currents and myofilament activation.

Cardioprotection associated with preconditioning in the anesthetized ferret

The cardioprotective effect of ischemic preconditioning (PC) was investigated in the anesthetized ferret model of myocardial ischemia followed by reperfusion. PC of 2, 5, or 10-min duration, followed by 10-min reflow, was studied in animals subjected to 60-min sustained LAD coronary artery ischemia followed by 5-h reperfusion. Infarct size was determined by tetrazolium staining. Sham PC ferrets had a mean infarct of 72% of risk zone. A 2-min or 5-min cycle of PC significantly reduced tissue damage to 54% (p < 0.05) and 44% (p < 0.01), respectively. Infarct reduction associated with 10-min ischemic PC was not significant (57% of AAR). The cardioprotective effects of 5-min PC were lost when sustained ischemia was prolonged to 75 or 90-min. Myocardial salvage afforded by 5-min PC was also abolished by both a) inhibition of ATP-sensitive potassium channels using either glyburide or 5-HD and b) blockade of adenosine receptors with the A1 selective agent DPCPX. In the absence of PC, activation of ATP-sensitive potassium channels with the cardiac-selective agonist BMS-180448 significantly (p < 0.01) reduced infarct size from 66% to 37% of the risk zone. Cardioprotection, or its loss, was not the result of hemodynamic alterations occurring during PC, drug administration, or the coronary occlusion and reperfusion phases. Based upon its body size and lack of extensive myocardial collateral circulation the ferret offers a usefull alternative small species for study of ischemia and reperfusion salvage. It is concluded in the ferret that: a) the threshold for PC is less than in either the rat, rabbit, or dog; unlike the dog and pig, the beneficial effects of PC are b) reduced when the ischemic PC interval is extended to 10-min or c) lost if sustained coronary occlusion is maintained for a period of 75-min or longer; and last, a role in PC for both d) ATP-sensitive potassium channels and e) adenosine A1 receptors can be demonstrated.