Objective: The aim was to investigate the steps in the excitation-contraction coupling process by which chronic exposure to cyclosporin A (cyclosporin) affects twitch force development by rat cardiac trabeculae.
Methods: The interval dependence and [Ca2+]o dependence of twitch force development by intact trabeculae isolated from myocardium of untreated rats and rats treated with cyclosporin (15 mg.kg-1.d-1 for 21 d) were studied in Krebs-Henseleit solution (K-H; pH 7.4 and 25 degrees C) and the force-pCa relation was examined in all trabeculae.
Results: The force-[Ca2+]o relation of cyclosporin treated trabeculae was shifted leftward compared to that of controls, but these trabeculae generated 35% less stress (force/cross sectional area) at optimal [Ca2+]o. Unlike control trabeculae, cyclosporin treated trabeculae showed spontaneous activity at all diastolic intervals, even at low [Ca2+]o. Treated and control trabeculae generated the same maximum stress [control: 78.1 (SEM 7.7) mN.mm-2, cyclosporin treated: 70.2(7.4) mN.mm-2] in the presence of extracellular Sr2+ ions in the Krebs-Henseleit medium. Maximum stress observed in the presence of Sr2+ was similar to the stress generated by maximum activation of chemically skinned trabeculae in both groups [control: 70(4.6) mN.mm-2; cyclosporin treated: 73(6.2) mN.mm-2). The force-pCa relation of cyclosporin treated muscles and control muscles after skinning were also indistinguishable [control pCa50 = 5.56(0.04); cyclosporin treated pCa50 = 5.58(0.03)]. The twitch force-interval relation at 0.7 mM [Ca2+]o in intact control trabeculae revealed postrest potentiation with a maximum [equivalent to 70% of twitch force at optimal [Ca2+]o -61.0(2.1) mN.mm-2] at 100 s and subsequent rest depression. Under the same conditions, twitch force development by cyclosporin treated trabeculae was closer to optimal force [41.4(7.1) mN.mm-2] at all intervals, and rest potentiation was reduced. Pronounced rest potentiation (as well as postextrasystolic potentiation) was still observed in cyclosporin treated trabeculae at [Ca2+]o < 0.7 mM. Postextrasystolic potentiation was reduced at 0.7 mM [Ca2+]o in these trabeculae, but the rate of decay of postextrasystolic potentiation and the rate of relaxation of the twitch force were unaffected.
Conclusions: These results suggest that the changes in the sensitivity of intact rat myocardium to [Ca2+]o and in maximum force development induced by cyclosporin are not due to changes in myofilament properties. The increased twitch force development as well as the spontaneous activity at low [Ca2+]o may be due to facilitated Ca2+ release from the sarcoplasmic reticulum due to altered properties of the sarcoplasmic reticular Ca2+ release channel, as both are observed when twitch force is submaximal, suggesting that the sarcoplasmic reticulum was not overloaded with Ca2+. The decline in peak stress with cyclosporin at [Ca2+]o > approximately 1.0 mM can be explained on the basis of spontaneous release of Ca2+ during the interval between twitches which leaves less Ca2+ for release from the sarcoplasmic reticulum with each action potential.