Platelet cytosolic free calcium concentration in essential hypertension

Intraplatelet free [Ca2+] was studied in human primary hypertension using the fluorescent indicator Quin-2. The cytosolic free [Ca2+] was observed to increase in human platelets as a function of the external [Ca2+]. In the absence of added external calcium, no difference in [Ca2+]i was observed between platelets of hypertensive patients and normotensive subjects (101 +/- 5 nM vs 89 +/- 5 nM, n = 27 and 33). In the presence of 1 nM Ca2+ in the external medium, [Ca2+]i was significantly higher in hypertensive patients than those of normotensive subjects (254 +/- 16 nM vs 202 +/- 10 nM, n = 27 and 34, p less than 0.01). Since an increase in cytosolic free Ca2+ concentration has been proposed to stem from the enhanced intracellular Na+ concentration due to the Na+ pump inhibition, the effects on [Ca2+]i of ouabain and removal of external K+ ions were investigated. No effect of 10(-4)M ouabain was observed whereas [Ca2+]i rose from 198 +/- 8 to 273 +/- 29 (n = 6) in absence of external K+ ions. The increase of intracellular free [Ca2+] in platelets of essential hypertensives observed only in the presence of external Ca2+ thus reflects a structural membrane alteration leading to an increased influx and/or a limited extrusion capacity. Its relationship with cell Na+ handling remains unclear.