POTASSIUM EXCRETION - A STORY THAT IS EASY TO DIGEST

The aim of this article is to consider how a large quantity of potassi um (K+) can be excreted without the development of hyperkalemia. The h ypothesis will focus on interactions between K+ and HCO3- primarily wi thin the kidneys. It is speculated that the absorption of K+ from the gastrointestinal tract is accompanied by an initial addition of HCO3- to the body; this in turn could, via intrarenal events, promote the de livery of HCO3- to the cortical collecting duct (CCD), where interacti ons may permit the development of a very high rate of excretion of K+. To test one portion of this hypothesis, studies were performed in she ep because they normally consume approximately in-fold more K+ per kil ogram body weight than do humans. In the absence of a significant degr ee of anabolism, there is only a limited potential to shift K+ acutely into cells. Hence, an extremely large capacity to excrete K+ is requi red to avoid a severe degree of hyperkalemia. The excretion of K+ depe nds primarily on the ability to have a sustained rise in the (K) in ea ch liter of luminal fluid exiting the CCD to very high revels and to h ave a large number of liters of fluid exit the terminal CCD while anti diuretic hormone is acting. A reasonable approximation of this CCD flo w rate can be obtained by examining the osmole excretion rate when ADH acts. Because these sheep excreted 1,650 mosmol (2 L x 827 mosm/kg H2 O) per day, a minimum estimate for volume delivery out of the CCD is 5 to 6 L/day. Hence, the average (K+) in the terminal CCD must be close to 74 mmol/L to permit the observed rate of excretion of K+. In contr ast to other species, sheep maintained a high (K) in the CCD throughou t the 24-h period; moreover, every urine sample contained HCO3-.