THE POTENTIAL ROLE OF THE ADRENAL MEDULLA IN HEMODYNAMIC RESPONSES DURING GABA ACTIVATION OF RAT AREA POSTREMA

The goal of this study was to investigate the potential role of the adrenal medulla in the multiple cardiovascular alterations that occur during activation of GABA(A) subtype in the area postrema. Blood pressure, heart rate, renal, iliac blood flows and plasma catecholamines were monitored during nonaliter infusion of muscimol (10 nl/min/10min, 1ng/nl) in the area postrema of the rats. Responses in pentobarbital-anesthetized male Albino rats, either intact (CON) or acutely adrenalectomized (ADX), were compared. Muscimol infusion increased mean arterial pressure in a dose-dependent manner in CON, but in ADX group such pressor effect was significantly attenuated. Heart rate was not significantly affected by drug infusion in either group. Renal bed actively constricted with non-significant transient increase in renal blood flow in both groups. Iliac blood flow increased markedly in CON, but remained unchanged in ADX. Iliac resistance was increased significantly in both CON and ADX, but it was rapidly attenuated to control values in CON. Plasma catecholamine concentrations was significantly elevated by muscimol infusion. We conclude that activation GABA(A) subtype may augment sympathetic outflow to the adrenals causing catecholamine release that in turn produces iliac bed vasodilation, but renal bed vasoconstriction. In ADX, this dominant effect of adrenal sympathetic activation was acutely attenuated because of withdrawal of sympathetic vasomotor tone in these resistance beds.