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.

Pressure-flow relationships (P/F) in the renal circulation were determined in 62 swine, aged 1 day-2 mo, anesthetized with pentobarbital sodium. Aortic and inferior vena caval pressures and renal and femoral arterial flows were recorded. Blood gas composition and pH and body temperature were monitored. The P/F was first determined while perfusion pressure was decreased for 2 min at each pressure by suprarenal aortic occlusion. The left renal artery in 38 of these animals was then cannulated for in situ perfusion of the kidney with blood withdrawn from a carotid artery by a Masterflex pump. The P/F was subsequently determined by changing pump flow for 2 min at each flow while recording perfusion pressure. Records were analyzed for transient and steady-state effects. Readjustments in renal vascular resistance (RVR) were apparent within 5 s after changing pressure or flow. The RVR stabilized at a new level within 2 min. Graphs of steady-state data delineated an autoregulatory range in the P/F for animals as young as 2 wk of age. We conclude that renal blood flow autoregulation in this mammal is negligible at birth and develops progressively during the first postnatal month.

Pressure-flow relationships (P/F) in the small intestine or hindlimb circulation were used to examine autoregulation in 104 swine, aged 1 day to 2 months and anesthetized with pentobarbital. The P/F were obtained while perfusion pressure was decreased by compression of the abdominal aorta for 2 min at each pressure. Readjustments in vascular resistance stabilized within 2 min. Intestinal flow decreased with each decrease in pressure in swine 2 weeks old or younger, but was well maintained by 1 month of age. Femoral flow decreased with each decrease in pressure at all ages studied. Control experiments indicated that the observed vascular responses were independent of angiotension-II activity and were not reflex in origin.

Renal and cardiac effects of norepinephrine and dopamine were evaluated in swine aged 1 wk, 2 wk, and 6 mo. The swine were anesthetized with pentobarbital (20-30 mg/kg). Aortic pressure, right ventricular pressure and its first derivative, and heart rate were recorded, together with carotid and renal (RBF) arterial flows. Glomerular filtration rate (GFR) was determined by [14C]inulin clearance. After a control period, norepinephrine or dopamine was infused intravenously for 10-20 min before and then during another clearance period. After a second control period, the second catecholamine was infused. GFR increased in piglets given either catecholamine. Norepinephrine at equipressor doses (2.0 micrograms.kg-1.min-1 in piglets and 1.0 micrograms.kg-1.min-1 in mature swine) decreased RBF and increased renal resistance. Dopamine at equi-inotropic doses (10 micrograms.kg-1 min-1 in piglets and 20 micrograms.kg-1.min-1 in mature swine) increased RBF and decreased renal resistance only in mature swine. Infusions of dopamine at a low dose (5 micrograms.kg-1.min-1) also failed to increase RBF or decrease renal resistance in piglets. The results suggest that maturation of the mechanism of renal vasodilation by dopamine occurs later than that for vasoconstriction by norepinephrine.

The relative maturity of immediate intestinal circulatory responses to efferent splanchnic nerve stimulation for 20 s was tested in 34 piglets (1 day-1 mo old) under pentobarbital anesthesia (15-30 mg/kg). Aortic pressure, heart rate, and superior mesenteric arterial flow (measured by electromagnetic flow transducers) were recorded simultaneously. Intestinal vascular resistance was calculated as mean aortic pressure/mean arterial flow. Resistance increased in most animals during experimental inhibition of the carotid sinus baroreceptor reflex. Transection of the splanchnic nerve decreased mesenteric resistance in all animals. Nerve stimulation at 2 Hz was not effective in all of the youngest animals; at 3 Hz, mesenteric resistance increased in all animals. The latency of this response was shorter at each increase in stimulation frequency. The aortic pressure rise associated with splanchnic nerve stimulation was greater with each increase in frequency. This rise was attenuated or absent after adrenalectomy and during postganglionic fiber stimulation. The vasoconstrictor effect of splanchnic nerve stimulation was attenuated after alpha-adrenergic receptor blockade with phentolamine in a dose (0.25-0.75 mg/kg) that blocked vasoconstrictor effects of norepinephrine (0.5 micrograms/kg). We conclude that alpha-adrenergic mechanisms are functional, although not mature, and that sympathetic vasoconstrictor tone is present in the intestinal circulation of swine at birth.