Sarcoplasmic reticulum and nuclear envelope are one highly interconnected Ca2+ store throughout cardiac myocyte

Previous ventricular myocyte studies indicated that ryanodine receptors (RyRs) are in the sarcoplasmic reticulum (SR) and are critical in excitation-contraction coupling, whereas the inositol trisphosphate (InsP(3)) receptors are separately localized on the nuclear envelope (NucEn) and involved in nuclear Ca2+ signaling. Here, we find that both caffeine and InsP(3) receptor agonists deplete free [Ca2+] inside both SR and NucEn. Fluorescence recovery after photobleach (FRAP) was measured using the low-affinity Ca2+ indicator Fluo-5N trapped inside the SR and NucEn (where its fluorescence is high because [Ca2+] is approximate to 1 mmol/L). After Fluo-5N photobleach in one end of the cell, FRAP occurred, accompanied by fluorescence decline in the unbleached end with similar time constants (tau approximate to 2 minutes) until fluorescence regained spatial uniformity. Notably, SR and NucEn fluorescence recovered simultaneously in the bleached end. Ca2+ diffusion inside the SR-NucEn was also measured. SR Ca2+-ATPase was completely blocked but without acute SR Ca2+ depletion. Then caffeine was applied locally to one end of the myocyte. In the caffeine-exposed end, free SR [Ca2+] ([Ca2+] SR) declined abruptly and recovered partially (tau=20 to 30 seconds). In the noncaffeine end, [Ca2+](SR) gradually declined with a similar tau, until [Ca2+] SR throughout the cell equalized. We conclude that the SR and NucEn lumen are extensively interconnected throughout the myocyte. Apparent intrastore diffusion coefficients of Fluo-5N and Ca2+ were estimated (approximate to 8 mu m(2) sec(-1) and 60 mu m(2) sec(-1)). This rapid luminal communication may maintain homogeneously high luminal [Ca2+], ensuring a robust and uniform driving force for local Ca2+ release events from either SR or NucEn.