Mechanism of biphasic response of renal nerve activity during acute cardiac tamponade in conscious rabbits

Renal sympathetic nerve activity (RSNA) responses to acute cardiac tamponad e were studied in conscious rabbits with all reflexes intact (Int) or after either surgical sinoaortic denervation (SAD) or administration of intraper icardial procaine tip-Pro) or intravenous procaine (iv-Pro). In Int rabbits , the mean arterial pressure (MAP) remained relatively constant until the p ericardial volume reached 7.7 ml, whereas the RSNA increased to 226% [compe nsated cardiac tamponade (CCT)], then, at a pericardial volume of 9.3 ml, t he MAP fell sharply and RSNA decreased to 34% [decompensated cardiac tampon ade (DCT)]; 1 min after cessation of pericardial infusion, an intravenous i njection of naloxone resulted in increases in both MAP and RSNA. In SAD rab bits, RSNA did not alter throughout CCT and DCT, but increased on injection of naloxone. In ip-Pro rabbits, RSNA increased during CCT but did not decr ease during DCT, whereas, in iv-Pro rabbits, the RSNA response was similar to that in Int rabbits. These results indicate that RSNA responses to cardi ac tamponade are biphasic, with an increase during CCT and a decrease durin g DCT. Sinoaortic baroreceptors are involved in mediating the increase in R SNA, whereas cardiac receptors may be involved in mediating the decrease in RSNA. An endogenous opioid may he responsible for the decrease in RSNA see n during DCT.