Neurally mediated renal vasoconstriction during isometric muscle contraction in cats

The aim of this study was to determine if the reflex increase in renal sympathetic nerve activity (RSNA) during static (isometric) muscle contraction evokes renal vasoconstriction and decreases renal blood flow. RSNA, renal blood flow velocity, and arterial pressure were measured simultaneously during isometric contraction of the hindlimb triceps surae muscle in eight chloralose-anesthetized cats. A 1-min contraction was evoked by stimulating the peripheral ends of the cut L₇ and S₁ ventral roots. RSNA and mean arterial pressure (MAP) increased 41 ± 14% (SE) and 50 ± 10 mmHg during static contraction, whereas mean renal blood flow velocity (MRBV) decreased 14 ± 5%. Calculated renal vascular resistance increased 73 ± 20% during the contraction. The increase in RSNA preceded the decrease in MRBV by 20 s. Passive mechanical stretch of the muscle increased RSNA 21 ± 12% but did not alter MRBV. Renal denervation abolished the decrease in MRBV during isometric contraction but only attenuated the rise in MAP. Cutting the L₄- S₁ dorsal roots or muscle paralysis abolished the MRBV and MAP responses. Thus reflex stimulation of RSNA from the contracting muscle can induce renal vasoconstriction and decrease renal blood flow.