30 Effects of angiotensin II on cytosolic calcium releases in isolated ventricular myocytes from adult normal and hypertensive rats

Abstract

Methods: This study investigated the effects of angiotensin II (Ang II; 10-9-10-7mol/l) on calcium releases in ventricular myocytes from normal and renal hypertensive adult rats (Goldblatt two kidney one clip). Newly isolated myocytes were loaded with fluorescent indo-1/AM and studied at rest or under electrical stimulation. The variation of the ratio of indo-1 emission (405/480 nm) was taken as a measure of cytosolic calcium variation. Five parameters were investigated from each peak systolic indo-1 ratio before and after Ang II addition: amplitude variation, duration with analysis of a rise time and a fall time, and frequency of spontaneous calcium releases. Results: The following changes were observed: (1) in unstimulated myocytes exhibiting spontaneous contractile activity, an increase in the frequency of calcium transients, at 10-7mol/l, in normal cells (+157±27%, P<0.01) and at each Ang II concentration in hypertrophied cells (+79±31%, P<0.01; +82±25%, P<0.01; +285±50%, P<0.01 at 10-9, 10-8 and 10-7 mol/l); (2) in stimulated myocytes, prolongation of the duration of calcium transients, explained by the occurrence of calcium releases during fall time. In addition, 50% of myocytes exhibited spontaneous releases of calcium in the interstimulus interval. The increase in calcium transient duration was statistically significant, regardless of the Ang II concentration in hypeitrophied cells (+36±20%, P<0.05; +39±18%, P 0.01; +77±34%, P<0.01 at 10-9, 10-8, 10-7mol/l) and only at 10-7mol/l in normal cells (+68±22%, P<0.01). Similar results were observed in fall time. Conclusion: Thus, in both normal and hypertrophied myocytes, Ang II induced calcium release, and hypertrophied cells became increasingly sensitive to Ang II. This occurrence of calcium release is a possible cause of arrhythmias, known as ‘triggered activity'.