Reflex responses to reductions in functioning renal mass.

Both acute unilateral nephrectomy (AUN) and unilateral ureteral obstruction (UUO) result in an acute increase in cation excretion from the contralateral kidney. AUN results in reflex changes in systemic hemodynamics owing to an acute and transient increase in arterial pressure that activates carotid sinus baroreceptors and constitutes an afferent limb in the reflex; hemodynamic adjustments and increased cation excretion result. The reflex involves participation of the endogenous opioid system, with receptors located primarily in the central nervous system, and requires intact pituitary function because both hypophysectomy and pretreatment with large doses of dexamethasone prevent the postnephrectomy natriuresis. The natriuresis is closely correlated with an increase in the plasma concentration of the NH2-terminal fragment of the pituitary peptide precursor molecule proopiomelanocortin, which suggests that such a peptide could participate directly or indirectly in the postnephrectomy natriuresis. Surgical denervation of either the ipsilateral or the contralateral kidney markedly alters the response to AUN, which prevents the natriuresis and blunts the kaliuresis, and indicates a role for renal neural reflexes. Renorenal reflex pathways also mediate the response of the contralateral kidney to UUO, because denervation of either the ipsilateral (obstructed) or the contralateral kidney abolishes both the natriuresis and kaliuresis usually seen after UUO. This reflex also involves the endogenous opioid system, for it does not occur in rats receiving an i.v. infusion of the opiate receptor antagonist naloxone.