Urine flow, sodium excretion, mean arterial blood pressure and glomerular filtration rate (GFR) were detennined in the conscious unrestrained rat infused with hypotonic saline. The effects of vasopressin infused at 24 and 160 pmol/min and oxytocin infused at 30 and 200 pmol/min were determined. The lower doses of each hormone gave plasma concentrations within the physiological range whereas the higher doses produced plasma concentrations equivalent to those seen following dehydration.
Vasopressin produced dose-dependent antidiuretic and natriuretic responses. Hormone infused at both rates increased the clearance of sodium, but only the higher dose caused a significant increase in GFR. Fractional excretion of sodium was significantly elevated by both doses. Oxytocin produced dose-dependent diuretic and natriuretic responses. Again both rates of infusion increased the clearance of sodium, but only the higher dose caused a significant increase in GFR. The lower dose caused a significant increase in the fractional excretion of sodium.
It appears, therefore, that increases in GFR may have a role in the natriuretic response to both hormones. However, this response can also be seen when GFR remains unchanged. This fact, together with the observed increases in the fractional excretion of sodium, indicates that these hormones have additional tubular actions.
Journal of Endocrinology (1994) 141, 59–67