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°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⁺]. Treated and control trabeculae generated the same maximum stress [control: 78.1(SEM7.7) 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⁺] – 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⁺] > ∼ 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.

Cardiovascular Research 1993;27:1845-1854