Effect of Angiotensin II on Calcium Release Phenomena in Normal and Hypertrophied Single Cardiac Myocytes

The effects of angiotensin II (Ang II) (10-9 M to 10-7 M) on calcium releases were established in ventricular myocytes from normal and renal hypertensive adult rats. From each peak systolic indo-1 ratio (405 nm/480 nm), amplitude variation, duration (rise time and fall time), and frequency of spontaneous calcium releases were investigated on freshly isolated cardiomyocytes at rest or under electrical stimulation. The following changes were observed: (1) in spontaneous contracting myocytes, an increase in frequency of calcium transients at 10-7 M in normal cells (+157%, P < 0.05) and at whatever angiotensin II concentration in hypertrophied cells (10-9 M: +79%, P < 0.05; 10-8 M +82%, P < 0.01; 10-7 M: +285%, P < 0.01) with a greater sensitivity of hypertrophied cells to Ang II (P < 0.05 at 10-9 M, P <0.01 at 10-8 M). (2) In stimulated myocytes, a prolongation of the duration of calcium atransients at 10-7 M in normal cells (+68%, P < 0.01) and at 10-9 M, 10-8 M, 10-7 M in hypertrophied cells: (+36%, P < 0.05; +39%, P < 0.01; +77%, P < 0.01) with a greater sensitivity of hypertrophied myocytes (P < 0.05 at 10-9 M and 10-8 M). An increase in duration may be explained by the occurrence of calcium releases during the fall time of calcium transients. Thus, both in normal and hypertrophied myocytes, Ang II induced the occurrence of calcium releases with increased sensitivity of hypertrophied cells to Ang II. Such calcium releases are known to be a possible cause of arrhythmias termed "triggered activity".