The effects of cyclopiazonic acid on intracellular Ca2+ in aortic smooth muscle cells from DOCA-hypertensive rats

ABSTRACT

We tested the hypothesis that cyclopiazonic acid (CPA), an inhibitor of the sarcoplasmic reticulum (SR) Ca2+ -ATPase, increases intracellular Ca2+ concentration ([Ca2+]i) in aortic myocytes and that the increase in [Ca2+]i is higher in aortic cells from deoxycorticosterone acetate (DOCA)-hypertensive rats. Male Sprague-Dawley rats, 250-300 g, underwent uninephrectomy, received a silastic implant containing DOCA (200 mg/kg) and had free access to water supplemented with 1.0 per cent NaCl and 0.2 per cent KCl. Control rats were also uninephrectomized, received normal tap water, but no implant. Intracellular Ca2+ measurements were performed in aortic myocytes isolated from normotensive (Systolic blood pressure = 120 + 3 mmHg; body weight = 478 ñ 7 g, N = 7) and DOCA-hypertensive rats (195 ñ 1O mmHg; 358 ñ 16 g, N = 7). The effects of CPA on resting [Ca2+]i and on caffeine-induced increase in [Ca2+]i after [Ca2+]i depletion and reloading were compared in aortic cells from DOCA and normotensive rats. The phasic increase in [Ca2+]i induced by 20 mM caffeine in Ca2+ -free buffer was significantly higher in DOCA aortic cells (329 ñ 36 nM, N = 5) compared to that in normotensive cells (249 ñ 16 nM, N = 7, P<0.05). CPA (3 muM) inhibited caffeine-induced increases in [Ca2+]i in both groups. When the cells were placed in normal buffer (1.6 mM Ca2+, loading period), after treatment with Ca2+ -free buffer (depletion period), an increase in [Ca2+]i was observed in DOCA aortic cells (45 ñ 11 nM, N = 5) while no changes were observed in normotensive cells. CPA (3 muM) potentiated the increase in [Ca2+]i (l22 ñ 3O nM, N = 5) observed in DOCA cells during the loading period while only a modest increase in [Ca2+]i, (23 ñ 10 nM, N = 5) was observed in normotensive cells. CPA-induced increase in [Ca2+]i did not occur in the absence of extracellular Ca2+ or in the presence of nifedipine. These data show that CPA induces Ca2+ influx in aorta from both normotensive and DOCA-hypertensive rats. However, the increase in [Ca2+]i is higher in DOCA aortic cells possibly due to an impairment in the mechanisms that control [Ca2+]i. The large increase in [Ca2+]i in response to caffeine in DOCA cells probably reflects a greater storage of Ca2+ in the SR.