Relaxation abnormalities in single cardiac myocytes from renovascular hypertensive rats

In myocardial hypertrophy secondary to renovascular hypertension, the rate of intracellular Ca2+ concentration decline during relaxation in paced left ventricular (LV) myocytes isolated from hypertensive (Hyp) rats is much slower compared with that from normotensive (Sham) rats. By use of a novel liquid-crystal television-based optical-digital processor capable of performing on-line real-time Fourier transformation and the striated pattern (similar to 1-dimensional diffraction grating) of cardiac muscle cells, sarcomere shortening and relaxation velocities were measured in single Hyp and Sham myocytes 18 h after isolation. There were no differences in resting sarcomere length, percent of maximal shortening, time to peak shortening, and average sarcomere shortening velocity between Sham and Hyp cardiac cells. In contrast, average sarcomere relaxation velocity and half-relaxation time were significantly prolonged in Hyp myocytes. Contractile differences between Sham and Hyp myocytes detected by the optical-digital processor are confirmed by an independent method of video tracking of whole cell length changes during excitation-contraction. Despite the fact that freshly isolated myocytes contract more rigorously than 18-h-old myocytes, the relaxation abnormality was still observed in freshly isolated Hyp myocytes, suggesting impaired relaxation is an intrinsic property of Hyp myocytes rather than changes brought about by short-term culture. We postulate that reduced sarcomere relaxation velocity is a direct consequence of impaired Ca2+ sequestration-extrusion during relaxation in Hyp myocytes and may be responsible for diastolic dysfunction in hypertensive hypertrophic myocardium at the cellular level.