Splanchnic sympathetic nerve activity and circulating catecholamines in the hyperthermic rat

The mechanisms responsible for the initial rise in splanchnic vascular resistance with environmental heating are controversial, and those responsible for the subsequent fall in splanchnic resistance in the severely hyperthermic animal are unknown. Thus we examined the effect of environmental heating on plasma catecholamine concentration, splanchnic sympathetic nerve activity (SNA), and select blood chemistries. In one study, 25 male Sprague-Dawley rats (270-300 g) were assigned to one of five groups on the basis of their core temperature (Tc, 37, 39, 41, 43, or 44 degrees C) at death. Heart rate (HR), mean arterial pressure (MAP), and Tc were monitored during heat stress under alpha-chloralose anesthesia (12.5 mg.ml-1.h-1). At each predetermined Tc, an aortic blood sample was drawn and analyzed for mean plasma concentration of norepinephrine (NE), epinephrine (E), Na+, K+, and lactate. From 41 to 43 degrees C, NE and E rose significantly, and the animals became hyperkalemic and lactacidemic. In a separate study, we quantitated SNA from the greater splanchnic nerve during heat exposure of artificially respired animals anesthetized with pentobarbital sodium (50 mg/kg). MAP, splanchnic SNA, and Tc were recorded. Tc was elevated from 37.0 +/- 0.12 to 41.3 +/- 0.18 degrees C in 70 min by increase of ambient temperature to 38 degrees C in an environmental chamber. Splanchnic SNA was 54 +/- 8 spikes/s at a Tc of 37 degrees C and increased significantly as Tc exceeded 39 degrees C (P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)