Impact of neurally mediated antidiuretic effect relative to pressure diuresis during acute changes in sympathetic nerve activity

We examined urine excretion during primary acute sympathetic activation (PASA) in anesthetized Wistar-Kyoto rats. Since arterial pressure (AP) changes with sympathetic nerve activity (SNA) during PASA, urine excretion reflects a neurally mediated antidiuretic effect combined with an effect of pressure diuresis. We hypothesized that preventing AP changes under PASA would enable the direct estimation of the neurally mediated antidiuretic effect alone. We changed the isolated carotid sinus pressure stepwise from 60 to 180 mmHg and compared the relationship of normalized urine flow (nUF, urine flow normalized by body weight) versus SNA between conditions allowing and preventing baroreflex-mediated changes in the mean AP. The slope of the SNA-nUF relationship was [Formula: see text]nUF_var = 0.444 ± 0.074 μL·min^-1·kg^-1·%^-1 when the mean AP was variable, whereas it was [Formula: see text]nUF_fix = -0.143 ± 0.032 μL·min^-1·kg^-1·%^-1 when the mean AP was fixed at 100 mmHg (n = 7 rats). The slope associated with the effect of pressure diuresis alone, calculated as [Formula: see text]nUF_var - [Formula: see text]nUF_fix, was 0.586 ± 0.105 μL·min^-1·kg^-1·%^-1. Hence, the potency of the neurally mediated antidiuretic effect |[Formula: see text]nUF_fix|/([Formula: see text]nUF_var - [Formula: see text]nUF_fix) was 0.235 ± 0.014 relative to the effect of pressure diuresis under PASA. Our findings would aid an integrative understanding of the effects of renal hemodynamic and sympathetic modulations on urine output function.