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Department of Veterinary Preclinical Sciences, University of Liverpool.

1. Intracellular calcium concentration ([Ca2+]i) was measured in single myocytes isolated from either the cardiac ventricle or the mesenteric artery of the rat. 2. In both cardiac and smooth muscle, the application of caffeine produced an increase of [Ca2+]i which spontaneously decayed back to resting levels. In vascular smooth muscle cells, removal of caffeine produced a transient fall of [Ca2+]i to below the resting level. [Ca2+]i then returned to control levels. A transient undershoot of [Ca2+]i on removal of caffeine was also sometimes seen in cardiac cells. When the undershoot was absent in cardiac cells it could be induced by elevating [Ca2+]o. 3. In vascular smooth muscle cells noradrenaline increased [Ca2+]i and an undershoot of [Ca2+]i could be produced by its removal. In cardiac cells a small undershoot could sometimes be seen following the systolic Ca2+ transient produced by electrical stimulation. 4. In both cardiac and vascular cells the time constant 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 recovery of the undershoot was also considerably slower than that of the caffeine response. 5. If caffeine was removed before the rise of [Ca2+]i had fully decayed spontaneously then the magnitude of the undershoot was reduced. 6. It is suggested that the undershoot of [Ca2+]i on removal of caffeine results from refilling of the SR decreasing [Ca2+]i. The data from vascular cells can be fitted by this model if the dissociation constant, Kd, of the surface membrane Ca2+ pump for [Ca2+]i is about 1 microM. 7. Using the model, it is concluded from the ratio of the time constants shown above that the caffeine releasable content of the sarcoplasmic reticulum constitutes about 80% of total cellular calcium in both cardiac and smooth muscle.

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