Extracellular Na+ concentration ([Na+]e) significantly effects the regulation of myogenic tone in isolated blood vessels. We examined the effect of small changes in [Na+]e on simultaneous changes in stretch-activated myogenic tone in rabbit facial vein and 45Ca2+ unidirectional influx and net uptake. Decreasing [Na+]e from 150 to 120 mmol/l augmented myogenic tone (control: 3.15 +/- 0.27 mN, n = 22) by 89 +/- 29%, while raising [Na+]e to 165 mmol/l attenuated myogenic tone to 80 +/- 2% of control. Changes in myogenic tone induced by alterations in [Na+]e were not accompanied by proportional changes in 45Ca2+ net uptake. 45Ca2+ unidirectional influx per unit of wall force (10.2 +/- 1.0 pmol/mg per mN force, n = 22, control) was decreased to 6.1 +/- 0.6 pmol/mg per mN (n = 20, P < 0.05) and increased to 21.0 +/- 2.5 pmol/mg per mN (n = 14, P < 0.05) when [Na+]e was 120 or 165 mmol/l, respectively, suggesting that decreasing [Na+]e is related to an increased sensitivity to calcium. We conclude that, in the rabbit facial vein, the sensitivity of myogenic tone to changes in [Na+]e may reflect changes in the sensitivity of smooth muscle to Ca2+ through a change in mechanoreceptor sensitivity.