Harald Langberg

GLOMERULOTUBULAR BALANCE: RENAL BICARBONATE REABSORPTION VARIES WITH PLASMA pH

Publisher Summary It has been generally accepted that plasma CO2 concentration, rather than plasma pH, regulates renal bicarbonate reabsorption. It is well-known that respiratory acidosis stimulates bicarbonate reabsorption and that respiratory alkalosis reduces bicarbonate reabsorption. This chapter explains the effect of plasma pH on bicarbonate reabsorption by inducing respiratory and metabolic alkalosis, and respiratory acidosis consecutively in the dog. The particular significance with regard to glomerulotubular balance is that the difference in pH across the proximal tubular epithelium determines bicarbonate reabsorption also during variations in glomerular filtration rate (GFR). The chapter presents two sets of experiments carried out on dogs anesthetized by sodium pentobarbital and mechanically ventilated. In the first series, five dogs were expanded approximately 10% of body weight by saline infusion and ethacrynic acid was administered to inhibit autoregulation. GFR could be varied over a wide range by either raising systemic blood pressure by carotid constriction, or reducing renal perfusion pressure by constricting the aorta above the origin of the renal arteries. By varying GFR and hence the filtered load, bicarbonate reabsorption is compared at the same filtered load at different plasma pH. The chapter presents results that indicate that the permeability of the proximal tubules to bicarbonate is very low, whereas it is high for hydrogen ions. These results are consistent with a high reflection coefficient for bicarbonate.