Pithed rats were respired at a fixed rate of 54 cycles min-1 and with a ventilation volume of either 20 (control) or 10 ml kg-1. In these two preparations, the dose-response relationships for the systemic blood pressure responses to endothelin-1, administered i.v., were examined. Also, cardiac output, its distribution, tissue blood flows and vascular resistances were determined at both respiratory volumes in pithed rats given saline or during pressor responses to endothelin-1 (750 ng, i.v.). Finally, a comparison was made of the pressor responses to endothelin-1 in the blood perfused superior mesenteric arterial bed of pithed rats respired at 10 or 20 ml kg-1. 2. In control rats the systemic blood pressure responses to i.v. endothelin-1 were biphasic with an initial, transient (30 s) decrease in blood pressure followed by a well sustained pressor response. These responses were dose-dependent (the ED50 for the pressor response being 0.27 +/- 0.04 micrograms). The pressor effect of endothelin-1 was due to an increase in total peripheral resistance with no change in heart rate or cardiac output. This increased total peripheral resistance was due to vasoconstriction of the spleen, stomach, large intestine, small intestine and the pancreas/mesentery (in which it was most severe). Endothelin-1 also increased blood flow through the heart, lungs, liver, epididimides, fat and skin through redistribution of cardiac output to these vascular beds. 3. At the lower ventilation volume there was moderate acidosis, hypoxia and hypercapnia relative to those rats respired at 20 ml kg-1. With respiration at 10 ml kg-1, the pressor response to endothelin-1 was not sustained and, after oscillations in both blood pressure and heart rate, death occurred 15-20 min after administration. The pressor effect resulted from increases in cardiac output (due to increased stroke volume) and total peripheral resistance: the latter was caused by vasoconstriction in the stomach, small intestine, large intestine and pancreas/mesentery. Endothelin-1 increased blood flow through the heart, lungs, liver, kidneys, testes, fat and skin due to either an increase in cardiac output, redistribution of cardiac output or both. 4. Endothelin-1 induced dose-dependent pressor responses in the mesenteric bed in situ. At the lower ventilation volume the potency of endothelin-1 in this vascular bed was increased approximately two fold with the ED50 being 68 +/- 7 pmol compared to 113 +/- 15 pmol in the rats respired at 20 ml kg-1. 5. This study indicates that, in normoxic control pithed rats, the pressor response to endothelin-1 was due largely to vasoconstriction of the splanchnic vascular bed. In rats with moderate hypoxia, hypercapnia and acidosis, the pressor response was due to vasoconstriction of the gastrointestinal tract as well as an increase in cardiac output. Endothelin-1 induced profound vasoconstriction in the mesenteric bed of the pithed rat both in vivo and in situ. The potency of endothelin-1 on this bed in situ was doubled by lowering the ventilation volume. An increase in cardiac contractility and severe gastrointestinal vasoconstriction may be the initial events leading to the eventual toxic effect of endothelin-1 in the hypoxic pithed rat.
