High dietary salt alters arteriolar myogenic responsiveness in normotensive and hypertensive rats

We evaluated arteriolar myogenic responsiveness in normotensive, salt-loade d and hypertensive rats and investigated the potential influence of luminal blood flow or shear stress on myogenic responses under each of these condi tions. Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR) fed low -salt. (0.45%, LS) or high-salt (7%, HS) diets were enclosed in a ventilate d airtight box with the spinotrapezius muscle exteriorized for intravital m icroscopy. Dietary salt did not affect mean arterial pressure (MAP) in WKY, whereas MAP in SHR was significantly higher and augmented by dietary salt. In all groups, box pressurization caused similar increases in MAP that wer e completely transmitted to the arterioles. After these pressure increases, large arteriole diameters decreased by 0-30% and intermediate arteriole di ameters decreased by 21-27%. Arteriolar myogenic responsiveness was not dif ferent between WKY-LS and SHR-LS. Large arterioles in WKY-HS displayed an a ttenuated pressure-diameter relationship compared with that in WKY-LS. Larg e arterioles in SHR-HS displayed an augmented pressure-diameter relationshi p compared with that in SHR-LS. There were no correlations between resting flow or wall sheer rate and the magnitude of initial myogenic constriction in any group or vessel type. The capacity for sustained myogenic constricti on was unrelated to secondary decreases in flow (14-41%) or increases in wa ll shear rate (21-88%) in each group. We conclude that 1) dietary salt impa irs the myogenic responsiveness of large arterioles in normotensive rats an d augments the myogenic responsiveness of large arterioles in hypertensive rats, 2) hypertension does not alter arteriolar myogenic responsiveness in this vascular bed, and 3) flow- or shear-dependent mechanisms do not attenu ate myogenic responses in the intact arteriolar network of normal, salt-loa ded, or hypertensive rats.