MYOGENIC TONE OF RABBIT FACIAL VEIN AND POSTERIOR CEREBRAL-ARTERY IS INFLUENCED BY CHANGES IN EXTRACELLULAR-SODIUM

We examined the effect of small changes in extracellular Na+ concentration ([Na+](e)) on myogenic tone (MT) in isometrically mounted ring segments of the rabbit facial vein and in pressurized cannulated posterior cerebral artery segments. Decreasing [Na+](e) from 150 to 120 mM in the vein increased MT by 24%, and raising [Na+](e) to 165 mM attenuated it by 30%. In pressurized posterior cerebral arteries, decreasing [Na+](e) to 120 mM reduced the intraluminal diameter by 12%, whereas increasing [Na+](e) to 165 mM increased it by 17%. MT was inhibited by amiloride [50% inhibitory concentration (IC50) = 17 +/- 6 mu M], an inhibitor of Na+-H+ exchange. Diisothiocyanatostilbene sulfonic acid, a Na+-Cl--HCO3- cotransporter blocker, inhibited MT with an IC50 of 4.4 +/- 0.65 mu M. Ouabain increased MT [50% effective concentration (EC(50)) = 0.10 +/- 0.04 mu M] as did the reintroduction of HCO3- (EC(50) 50 +/- 1.5 mM). Our study suggests that MT in the rabbit posterior cerebral artery and rabbit facial vein is modulated by changes in [Na+](e). This effect is independent of the method used to register changes in wall force. The sensitivity of the tone to changes in [Na+](e) and the independence of vessel diameter at different pressures at various [Na+](e) may reflect changes in the sensitivity of smooth muscle stretch or mechanoreceptors to [Na+](e).