Calcium handling by vascular myocytes in hypertension

Calcium ions (Ca2+) trigger the contraction of vascular myocytes and the level of free intracellular Ca2+ within the myocyte is precisely regulated by sequestration and extrusion mechanisms. Extensive evidence indicates that a defect in the regulation of intracellular Ca2+ plays a role in the augmented vascular reactivity characteristic of clinical and experimental hypertension. For example, arteries from spontaneously hypertensive rats (SHR) have an increased contractile sensitivity to extracellular Ca2+ and intracellular Ca2+ levels are elevated in aortic smooth muscle cells of SHR. We hypothesize that these changes are due to an increase in membrane Ca2+ channel density and possibly function in vascular myocytes from hypertensive animals. Several observations using various experimental approaches support this hypothesis: 1) the contractile activity in response to depolarizing stimuli is increased in arteries from hypertensive animals demonstrating increased voltage-dependent Ca2+ channel activity in hypertension; 2) Ca2+ channel agonists such as Bay K 8644 produce contractions in isolated arterial segments from hypertensive rats and minimal contraction in those from normotensive rats; 3) intracellular Ca2+ concentration is abnormally increased in vascular myocytes from hypertensive animals following treatment with Ca2+ channel agonists and depolarizing interventions, and 4) using the voltage-clamp technique, the inward Ca2+ current in arterial myocytes from hypertensive rats is nearly twice as large as that from myocytes of normotensive rats. We suggest that an alteration in Ca2+ channel function and/or an increase in Ca2+ channel density, resulting from increased channel synthesis or reduced turnover, underlies the increased vascular reactivity characteristic of hypertension.

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

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.