Humoral factor, intraplatelet calcium, and hypertension.

Plasma humoral factors which modulate the transmembrane distribution of sodium and calcium have been identified in hypertensive patients and have been hypothesized to be involved in the etiology of essential hypertension. In cross-incubation experiments, we have found that plasma of hypertensive subjects elevated basal and stimulated intraplatelet calcium levels, while plasma of normal subjects has an opposite effect on platelets from hypertensives. Basal intraplatelet calcium in normal platelets was 108 +/- 5 nmol/L and rose to 142 +/- 3 nmol/L (P less than .001) after incubation in plasma from hypertensive patients. Platelets from hypertensive patients had basal calcium levels of 182 +/- 11 nmol/L which fell to 127 +/- 11 nmol/L (P less than .01) after incubation in normal plasma. Hypertensive plasma potentiated the rise in intraplatelet calcium in response to ADP and PAF. Hypertensive patients treated experimentally with plasmapheresis exhibited a disappearance of the plasma factor responsible for elevating intraplatelet calcium. These results indicate the presence of a plasma humoral factor in hypertensives which elevates intraplatelet calcium and sensitizes platelets to agonist stimuli.