Pregnant rats became significantly heavier than non-pregnant controls 2 days after conception. Just prior to term, they exhibited a gross 52% increase over their non-pregnant body weight. Dry kidney weight had increased by 13% in 7-day-pregnant animals, which also showed a 20% increase in proximal renal tubule lengths. At term, the former was not maintained as compared with controls; whereas the latter is maintained throughout pregnancy, and may, at least partially, account for the enhanced renal reabsorption of salt and water found in these animals.
The effects of changes in luminal flow rate on fluid absorption in rat renal proximal convoluted tubules were studied by continuous luminal and peritubular microperfusion methods. Luminal flow rate was varied over a range from 5 to 45 nl . min-1, and the effects of transepithelial chloride and bicarbonate gradients were tested. Fluid absorption across the proximal convoluted tubule increased with luminal flow rate in the absence of luminal bicarbonate and organic solutes but in the presence of transepithelial chloride and bicarbonate gradients and active sodium transport. Augmenting perfusion rate from 5 to 45 nl . min-1 resulted in an increase of volume absorption from 0.49 to 3.37 nl . min-1 per millimeter length of tubule-1. The chloride concentration change in the collected perfusate decreased from 5.9 to 2.6 mEq . liter-1 . mm length tubule-1 over the same perfusion range. Thus, tubular chloride concentration rises with perfusion rate such that the steepest transepithelial chloride gradients are maintained at the highest flow rates. Flow dependence continued, albeit at reduced rate, in the absence of active sodium transport (cyanide perfusion) but in the presence of chloride and bicarbonate gradients. Flow dependence disappeared in the absence of both active sodium transport and transepithelial anion gradients. Luminal and peritubular perfusion experiments with symmetrical bicarbonate-free solutions that contained only phosphate buffer showed that even under those conditions fluid movement driven by cyanide-sensitive active transport increased with flow rate.
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