Mechanical performance of the working rat heart: Response to calcium

Abstract

Although calcium is generally considered a substance that can enhance cardiac pump performance, few data exist to demonstrate such effect when plasma ionized calcium is varied in the clinical concentration range. We have studied the relationship between perfusate ionized calcium concentration ([Ca 2+]) and myocardial mechanical performance in the isolated, blood perfused, ejecting rat heart. With pre- and afterload near constant, observations were made before and after steadystate increases in [Ca 2+] from low (mean ± SEM: 0.74 ± 0.01 m M) to normal (1.01 ± 0.04 m M); from low (1.01 ± 0.02 m M) to higher than normal (1.45 ± 0.04 m M, i.e., within the clinical hypercalcemia range); and to an excessive value (1.94 ± 0.03 m M). When [Ca 2+] was increased from low to normal levels, aortic blood flow and stroke volume increased by approximately 25% of control ( P < 0.025); heart rate did not change significantly. In contrast, when [Ca 2+] was raised from 1.03 m M (normal value) to 1.45 m M (within the clinical hypercalcemia range) or to 1.94 m M (above the clinical hypercalcemia range), the changes in aortic flow and stroke volume were less ( P < 0.05) as compared to those observed when ionized calcium was adjusted from a level below normal. We conclude that the isolated, blood-perfused heart with pre- and afterload near constant, responds to sustained increases in perfusate ionized calcium with increased aortic flow and stroke volume, but that these changes are disproportionally small when ionized calcium is elevated to above normal from a normal baseline value.