RELAXATION IN FERRET VENTRICULAR MYOCYTES - UNUSUAL INTERPLAY AMONG CALCIUM-TRANSPORT SYSTEMS

1. Transport systems responsible for removing Ca2+ from the myoplasm during relaxation in isolated ferret ventricular myocytes were studied using caffeine-induced contractures. Internal calcium concentration ([Ca2+](i)) was measured with the fluorescent calcium indicator indo-1, and the results were compared with our recent detailed characterizations in rabbit and rat myocytes. 2. Relaxation and [Ca2+](i) decline during a twitch in ferret myocytes were fast and similar to that in rat myocytes (i.e. half-time, t(1/2) approximate to 100-160 ms). 3. During a caffeine-induced contracture (SR Ca2+ accumulation prevented), relaxation was still relatively fast (t(1/2) = 0 .57 s) and similar to relaxation in rabbit supported mainly by a strong Na+-Ca2+ exchange. 4. When both the SR Ca2+ uptake and Na+-Ca2+ exchange are blocked (by caffeine and 0 Na+, 0 Ca2+ solution) relaxation in the ferret myocyte is remarkably fast (similar to 5-fold) compared with rabbit and rat myocytes. The decline of the Ca-1(2+) transient was also fast under these conditions. These values were similar to those in rat under conditions where relaxation is due primarily to Na+-Ca2+ exchange. 5. Additional inhibition of either the sarcolemmal Ca2+-ATPase or mitochondrial Ca2+ uptake caused only modest slowing of the relaxation of caffeine-induced contracture in 0 Na+, 0 Ca2+ (t(1/2) increased to similar to 3 S). In rabbit myocytes the relaxation t(1/2) is slowed to 20-30 s by these procedures. 6. Even when the systems responsible for slow relaxation in rabbit ventricular myocytes are inhibited (i.e. sarcolemmal Ca2+-ATPase and mitochondrial Ca2+ uptake) along with the SR Ca2+-ATPase and Na+-Ca2+ exchange, relaxation and [Ca-2+(])(i) decline in ferret myocytes remain rapid compared with rabbit myocytes. 7. Ca2+ taken up by mitochondria in rabbit myocytes during a caffeine contracture in 0 Na+, 0 Ca2+ solution gradually returns to the SR after caffeine removal, but this component appears to be much smaller in ferret myocytes under the same conditions. 8. We tested for possible residual Ca2+ transport by each of the four systems which suffice to explain Ca2+ removal from the cytoplasm in rabbit (SR Ca2+-ATPase, Na+-Ca2+ exchange, sarcolemmal Ca2+-ATPase and mitochondrial Ca2+ uptake). We conclude that there is an additional calcium transport system at work in ferret myocytes. For this additional system, our results are most compatible with a trans-sarcolemmal Ca2+ transport, but neither a cation exchanger nor a Ca2+-ATPase with characteristics like that in other cardiac cells. This additional system appears able to transport Ca2+ nearly as fast as the Na+-Ca2+ exchange in rat ventricular myocytes.