Inner medullary hemodynamics in dogs with aortocaval fistula

To investigate the role of medullary hemodynamics and vasoactive hormones in sodium retention in dogs with aortocaval fistula, we examined papillary plasma flow (PPF), solute content, and renal output of renin, norepinephrine, and prostaglandin E2 (PGE2) in anesthetized normal and fistula dogs. During hydropenia, cardiac output was elevated and systemic vascular resistance reduced in fistula dogs, accompanied by markedly increased renal output of renin, norepinephrine, and PGE2. In fistula dogs the blunted diuretic and natriuretic response to saline loading was not due to impaired myocardial contractility. During hydropenia and after saline loading, glomerular filtration rate (GFR) and renal blood flow were similar in normal and in fistula dogs; however, PPF was significantly lower in fistula dogs, accompanied by significantly greater papillary tissue osmolality and sodium content. These findings indicate that in fistula dogs enhanced medullary sodium reabsorption is associated with decreased PPF and stimulation of the renin-angiotensin and adrenergic nervous system. Furthermore, the reduced PPF obviates medullary solute washout during saline loading, and may contribute to the blunted diuretic and natriuretic response.