The relationship between the plasma potassium concentration and renal potassium excretion in the adrenalectomized rat

The purpose of this study was to evaluate the renal mechanisms which lead to a high urine [K⁺] in adrenalectomized (ADX) rats devoid of aldosterone. By dividing the urine [K⁺] by the urine to plasma osmolality ratio, the [K⁺] in the cortical collecting duct luminal fluid can be estimated; dividing this value by the plasma [K⁺] yields an index of the transtubular [K⁺] gradient (TTKG) in vivo. The TTKG was close to 7 in aldosterone deficient ADX rats while on a normal K⁺ diet and fell towards unity when amiloride or a low K⁺ diet was administered to these rats. With a longer time on a low K⁺ diet, the TTKG was less than 1 in ADX rats. This suggests that K⁺ was reabsorbed in the medullary collecting duct under these conditions. Hyperkalaemia appears to have an 'aldosterone-like' action in the cortical collecting duct in vivo in the absence of aldosterone in ADX rats. This action of hyperkalaemia permits normal K⁺ excretion rates despite the absence of mineralocorticoids.