Figure 1. Effect of extracellular Na⁺ reduction on arteriolar myogenic responsiveness.

A (n = 13) illustrates that reduction of Na⁺ causes a concentration-dependent vasoconstriction (diameter expressed relative to passive diameter at 70 mmHg), which is associated with an increase in global intracellular Ca²⁺ (B; n = 13). Changes in Ca²⁺ are expressed as the fold change in 340/380 nm fluorescence ratio relative to that at 137 mm extracellular Na⁺. C and D, the effect of Na⁺ reduction on the steady-state responses to a 50–120 mmHg pressure step. C represents intact arterioles (n = 6) while D shows data for de-endothelialized vessels (n = 6) illustrating that responses are at the level of the vascular smooth muscle. E, the pressure step data expressed in terms of the myogenic index (see text). Myogenic index declines with extracellular Na⁺ concentration in both intact (n = 6) and de-endothelialized (n = 6) vessels. F, that vasoconstriction with KCl (30 or 60 mm) does not significantly alter myogenic index (n = 4) suggesting the data in E are not simply a result of an alteration in starting diameter. Myogenic responsiveness was determined before and after washout of KCl as a control for time. Results are expressed as means ± s.e.m.; ***P < 0.001.