Mechanisms of methyclothiazide-induced inhibition of contractile responses in rat aorta.

Methyclothiazide (MCTZ), a thiazide diuretic, inhibits the contractile response induced by norepinephrine in aortic rings from 12-week-old spontaneously hypertensive rats (SHR). Although not modified by indomethacin, this inhibition was attenuated by either mechanical removal of the endothelium or N omega-nitro-L-arginine (NOLA) treatment. These results suggest that the MCTZ effects on the norepinephrine-evoked vascular response are mediated by an endothelium-dependent mechanism involving endothelium-dependent relaxing factor (EDRF)/nitric oxide (NO) release. MCTZ was also found to alter the contractile response induced by the addition of Ca(2+) to a depolarizing solution, and this inhibitory effect was partially abolished by NOLA application. Our data led us to propose that MCTZ relaxes aortic rings, resulting in an endothelium-dependent relaxation phenomenon that could even be reinforced under high-K(+) depolarizing conditions.