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

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We estimated the time taken for repletion of the junctional sarcoplasmic reticulum (JSR) Ca²⁺ 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), in order to evaluate abnormality in Ca²⁺ handling by cardiac JSR in hypertension. There were no differences in contractility or in the time course of mechanical restitution between SHR and the control (WKY) at 3 weeks of age. Seven and 20-week-old SHR showed a greater rested state contraction (RST) and similar or smaller rapid cooling contracture compared with the control, suggesting that their JSR contains a similar amount of Ca²⁺ at saturation and releases more Ca²⁺ upon stimulation, compared to the control. The adult SHR and control showed similar mechanical restitution time courses, but the adults had longer pretwitch latency. The function G(t) representing the time course of JSR Ca²⁺ store repletion in adult SHR exceeded the control value at t ≤ 0.5 s, while the function H(t) representing JSR [Ca²⁺] change corresponding to the mechanical restitution after RST was smaller in the adult SHR at t ≤ 0.5 s, resulting in smaller H(t)/G(t), in adult SHR at t ≤ 0.5 s. Deviations of G(t), H(t) and H(t)/G(t) from control were greater at 20 than at 7 weeks. The results suggest acceleration of JSR Ca²⁺ store repletion and alternation of the Ca²⁺-induced release of Ca²⁺ from the JSR in young adult SHR.