Timing of the onset of changes in renal energetics in relation to blood pressure and glomerular filtration in haemorrhagic hypotension in the rat.

The timing and circumstances of changes in renal energetics during the gradual induction of haemorrhagic hypotension were studied in anaesthetised rats by phosphorus-31 nuclear magnetic resonance. Animals were bled at a constant rate of 0.1 ml/min via the femoral artery. Whenever changes in renal energetics were seen, a similar pattern was observed. A decrease in adenosine triphosphate occurred rapidly and was always associated with accumulation of inorganic phosphate and tissue acidosis. Profound oliguria, reflecting a markedly decreased rate of glomerular filtration preceded the changes in metabolite levels. Such a fall in glomerular filtration rate and consequently in the energy requirement for tubular reabsorption could be viewed as a mechanism by which energy demands of the kidney are reduced before a critical limitation of energy supply is reached. During uncomplicated haemorrhage in Wistar rats, mean arterial pressures as low as 25-40 mm Hg were reached before changes in renal energetics developed. In contrast, spontaneously hypertensive rates subjected to uncomplicated haemorrhage, and Wistar rats subjected to haemorrhage during concurrent stimulation of the ipsilateral sciatic nerve, developed changes in renal energetics at higher and more variable blood pressures and in response to the withdrawal of lesser but more variable quantities of blood. The sudden onset and severe degree of energy depletion at varying blood pressures during bleeding and its more ready occurrence in animals in which sympathetic nervous activity could be expected to be increased, suggests that sudden renal vasoconstriction is responsible for the unpredictable occurrence of tubular ischaemia in haemorrhagic hypotension.