Effects of cytosolic Ca2+ on the Ca2+ content of the sarcoplasmic reticulum in saponin-permeabilized rat ventricular trabeculae

 Rat ventricular trabeculae were mounted for isometric tension recording, and then permeabilized with saponin. The Ca2+ concentration ([Ca2+]) within the permeabilized preparation (cytosolic [Ca2+]) was monitored continuously using Indo-1 and the integrals of Ca2+ transients resulting from brief caffeine application used as an index of the sarcoplasmic reticulum (SR) Ca2+ content. The relationship between SR Ca2+ content and cytosolic [Ca2+] was studied within the reported physiological range (i.e. 50–250 nmol · l–1 Ca2+). Increasing cytosolic [Ca2+] from 50 nmol · l–1 to 250 nmol · l–1 increased the steady-state SR Ca2+ content about threefold. However, increasing [Ca2+] above 250 nmol · l–1 typically resulted in spontaneous SR Ca2+ release, with no further increase in SR Ca2+ content. The SR Ca2+ content increased only slowly when cytosolic [Ca2+] was increased; it was unchanged 20 s after a rapid increase in cytosolic [Ca2+], but increased progressively to a new steady-state level during the following 1–2 min. In a parallel series of experiments using intact papillary muscles, increasing extracellular [Ca2+] (from 0.5 to 5 mmol · l–1) significantly increased twitch tension within 20 s of the solution change. These results support previous suggestions that the SR Ca2+ content may increase when diastolic cytosolic [Ca2+] rises during inotropic interventions such as increased stimulus rate or extracellular [Ca2+]. However, the rate at which SR Ca2+ responds to changes in cytoplasmic [Ca2+] within the diastolic range does not appear rapid enough to explain the early potentiation of twitch tension in intact preparations after an increase in extracellular [Ca2+].