CHANGES OF INTRACELLULAR [CA2-RETICULUM IN RAT VENTRICULAR AND VASCULAR SMOOTH-MUSCLE(] DURING REFILLING OF SARCOPLASMIC)

1. Intracellular calcium concentration ([Ca2+]i) was measured in singl e myocytes isolated from either the cardiac ventricle or the mesenteri c artery of the rat. 2. In both cardiac and smooth muscle, the applica tion of caffeine produced an increase of [Ca2+]i which spontaneously d ecayed back to resting levels. In vascular smooth muscle cells, remova l of caffeine produced a transient fall of [Ca2+]i to below the restin g level. [Ca 2+]i then returned to control levels. A transient undersh oot of [Ca 2+]i on removal of caffeine was also sometimes seen in card iac cells. When the undershoot was absent in cardiac cells it could be induced by elevating [Ca2+]o. 3. In vascular smooth muscle cells nora drenaline increased [Ca 2+]i and an undershoot of [Ca 2+]i could be pr oduced by its removal. In cardiac cells a small undershoot could somet imes be seen following the systolic Ca2+ transient produced by electri cal stimulation. 4. In both cardiac and vascular cells the time consta nt of decay of the caffeine response (tau(caff)) was less than that of the recovery from the undershoot (tau(us)). On average the ratio tau( us):tau(caff) was about 5 in smooth muscle. In cardiac cells the recov ery of the undershoot was also considerably slower than that of the ca ffeine response. 5. If caffeine was removed before the rise of [Ca 2+] i had fully decayed spontaneously then the magnitude of the undershoot was reduced. 6. It is suggested that the undershoot of [Ca2+]i on rem oval of caffeine results from refilling of the SR decreasing [Ca2+]i. The data from vascular cells can be fitted by this model if the dissoc iation constant, K(d), of the surface membrane Ca 2+ pump for [Ca 2+]i is about 1 mum. 7. Using the model, it is concluded from the ratio of the time constants shown above that the caffeine releasable content o f the sarcoplasmic reticulum constitutes about 80% of total cellular c alcium in both cardiac and smooth muscle.