Renal circulatory effects of adrenergic stimuli in anesthetized piglets and mature swine

The relative maturity of renal circulatory responses to efferent renal nerve stimulation, and to exogenous norepinephrine and isoproterenol, was tested in 62 piglets (1--16 days old) under pentobarbital anesthesia (10--25 mg/kg). Aortic pressure, heart rate, and renal and femoral arterial flows (measured by electromagnetic flow transducers) were recorded simultaneously. Renal vascular resistance was calculated as mean aortic pressure/mean flow. Transection of the renal nerve resulted in decreased renal resistance in all animals. Efferent renal nerve stimulation at increasing frequencies (2--12.5 Hz, at 1.2 ms pulse duration and 1.0 mA current) showed age-dependent differences in the threshold and also in the magnitude of increase in renal resistance. Norepinephrine (0.05--1.0 microgram/kg) caused age-dependent increases in renal resistance. Restoration of renal flow toward control level occurred during the peak pressor effect of norepinephrine only in older piglets. Isoproterenol (0.05--1.0 microgram/kg) did not alter renal resistance consistently in piglets younger than 1 wk. Phentolamine (0.25 mg/kg) attenuated or blocked resistance increases to 0.5 microgram norepinephrine/kg or to renal nerve stimulation at 12.5 Hz in all animals. Propranolol (0.1 mg/kg) attenuated or blocked resistance decreases to 0.1 microgram isoproterenol/kg, which occurred only in older piglets. These results indicate the presence of an active alpha-adrenergic vasoconstrictor mechanism and absence of the beta-adrenergic vasodilator mechanism in the renal circulation of swine at birth.