Acceleration of Ca2+ Repletion in the Junctional Sarcoplasmic Reticulum and Alternation of the Ca2+-Induced Ca2+-Release Mechanism in Hypertensive Rat (SHR) Cardiac Muscle

We estimated the time taken for a repletion of the junctional sarcoplasmic reticulum (JSR) Ca(2+) stores from a family of mechanical restitution curves after twitches of various magnitudes in the cardiac muscle of hypertensive rats (SHR), using a method described previously (Tameyasu et al. Jpn J Physiol. 2004;54:209-19), to evaluate abnormality in Ca(2+) handling by cardiac JSR in hypertension. We found no differences in contractility or in the time course of mechanical restitution between SHR and the controls (WKY) at 3 weeks of age. In comparison to WKY, 7- and 20-week-old SHR showed a greater rested state contraction (RST) and similar or smaller rapid cooling contracture, suggesting that their JSR contains a similar amount of Ca(2+) at saturation, but releases more Ca(2+) upon stimulation. The adult SHR and WKY showed similar mechanical restitution time courses, but the adults had longer pretwitch latencies. The function G(t) representing the time course of JSR Ca(2+) store repletion in adult SHR exceeded the WKY value at t < or = 0.5 s, but the function H(t) representing JSR [Ca(2+)] change corresponding to the mechanical restitution after RST was smaller in the adult SHR at t < or = 0.5 s, resulting in smaller H(t)/G(t) in adult SHR at t < or = 0.5 s. Deviations of G(t), H(t), and H(t)/G(t) from WKY were greater at 20 weeks than at 7. The results suggest an acceleration of JSR Ca(2+) store repletion and an alternation of the Ca(2+)-induced release of Ca(2+ )from the JSR in young adult SHR.