PROSTACYCLIN, THROMBOXANE-A2, AND HYPERTENSION

Prostacyclin and thromboxane A2, products of separate branches of the arachidonic acid cascade, can have opposing effects on kidney function and on the vascular musculature. Prostacyclin acts as a vasodilator while thromboxane A2 has a vasoconstrictor effect and the balance between these two compounds appears to contribute to the homeostatic regulation of normal blood pressure. In the hypertensive state, this balance is disrupted and, at least in animal models of hypertension, there is excessive production of both. The increase in prostacyclin formation may be a reaction to the elevated blood pressure, possibly due to mechanical stimulation of the vascular smooth muscle cells in the blood vessel wall. However, the increase in thromboxane A2 may be more directly involved in the development and maintenance of hypertension. Not only is thromboxane A2 a vasoconstrictor but it can also stimulate the growth and proliferation of vascular smooth muscle cells which may account for the vascular hypertrophy seen in hypertension. Both of these actions would increase total peripheral resistance and contribute to hypertension. Whether prostacyclin and thromboxane A2 are in fact involved as causative agents in essential hypertension must await future research.