Pregnancy is associated with changes in renal haemodynamics and electrolyte handling. Recent evidence suggests that the ovarian hormone relaxin (RLX) may contribute to these changes. Infusion of RLX for 5 days resulted in a 20-40 % increase in glomerular filtration rate (GFR) and effective renal plasma flow in conscious male rats (Danielson et al. 1999). However, this paper did not report the effects of RLX on electrolytes, hence the aim of this study was to determine whether RLX alters renal electrolyte handling.
Under isoflurane anaesthesia, male Sprague-Dawley rats were implanted with an osmotic minipump (Alzet model 2001) loaded with either recombinant human RLX (rhRLX, 4 mg h-1, n = 9, a gift from Dr E. Unemori, Connetics Corp., USA) or vehicle (20 mM sodium acetate, n = 7). After 7 days animals were prepared for renal function study. Under Intraval anaesthesia (100 mg kg-1 sodium thiopentone) rats received euvolaemic fluid replacement (0.9 % saline, [3H]inulin, para-aminohippuric acid) of spontaneous urine output using a servo-controlled system (Burgess et al. 1993). After a 3 h equilibration period, urine and plasma samples were taken over a further 3 h. Animals were humanely killed at the end of the experiment.
After 7 days administration of rhRLX effective renal blood flow (ERBF) and urine flow rate were significantly higher than in control rats (P < 0.05, ANOVA), but GFR was unaltered. Sodium excretion tended to be higher, while potassium excretion tended to be lower in rhRLX-treated rats, but these differences did not achieve statistical significance. However the Na+:K+ ratio was significantly increased (P < 0.05). Plasma sodium and potassium concentrations and plasma osmolality were significantly lower (P < 0.05) in rhRLX-treated rats (Table 1).
rhRLX induced changes in renal function in male rats which resemble those seen in pregnancy, namely an increase in ERBF and urine output, coupled with a reduction in plasma sodium and osmolality. The lack of apparent effect on GFR may reflect the influence of anaesthesia. These data suggest that RLX may affect renal electrolyte handling in addition to its previously reported effects on haemodynamics.
All procedures accord with current UK legislation.