Tissue K⁺ loss from the perfused rat kidney: Effects of lactate and albumin treatment

Isolated rat kidneys lost approximately 22% of their tissue K⁺ when perfused with a Krebs Ringer bicarbonate medium containing a specially prepared substrate free albumin as the colloid. Experiments were done to determine if substrates removed (e.g. lactate, citrate, fatty acids) from the fraction V bovine serum albumin during its preparation were essential for maintenance of tissue K⁺. We measured tissue K⁺ in kidneys perfused with i) substrate free albumin, ii) substrate free albumin plus lactate, or iii) untreated albumin. The tissue K⁺ (μeq/g dry wt) of each perfused kidney was compared with the unperfused kidney from the same rat. While tissue K⁺ was not maintained when the kidneys were perfused with substrate free albumin (remaining tissue K⁺ = 78 ± 2%; n = 8) tissue K⁺ was maintained when the kidneys were perfused with either substrate free albumin plus lactate (98 ± 3%; n = 11) or with untreated albumin (94 ± 3%; n = 13). The cumulative K⁺ excretion was the same in all three groups. Lactate or substances associated with untreated albumin increased the fractional Na⁺ reabsorption. Lactate also increased glomerular filtration rate. Therefore, the isolated perfused kidney can be used to study the requirements for the maintenance of tissue composition in addition to the requirements for function.