Effect of maternal hypokalaemia on unidirectional maternofetal and net potassium fluxes across the placenta of the anaesthetized rat.

Potassium (K+) fluxes across the placenta of rats, at 21 days gestation, fed a low K+ diet or a control diet were studied. The rats on the low K+ diet had a significantly (P less than 0.001) lower arterial plasma K+ concentration compared to those on the control diet (1.95 +/- 0.12 and 2.93 +/- 0.06 mmol/l respectively; mean +/- s.e., n = 17). Fetal umbilical arterial plasma K+ concentration was unaltered in maternal hypokalaemia and was significantly (P less than 0.001) higher than that of maternal plasma (4.58 +/- 0.15 and 4.66 +/- 0.12 mmol/l in low K+ and control groups respectively). Net K+ flux across the placentas (as measured by fetal accretion between days 20 and 21 of gestation) of hypokalaemic mothers (0.106 +/- 0.02 mumol/min/g placenta, n = 6) was not different to that in controls (0.104 +/- 0.01 mumol/min/g placenta, n = 8). Unidirectional maternofetal flux (Jmf) across the placentas, measured as the accumulation of 42K in the fetuses after injection of the radioisotope into maternal blood, was also not significantly different between the hypokalaemic and control mothers (0.5 +/- 0.08 mumol/min/g placenta, n = 8 versus 0.63 +/- 0.06 mumol/min/g placenta, n = 7, respectively). However, measurement of Jmf by perfusion of the placentas through their fetal circulations yielded a higher value than the accumulation method and in this analysis was significantly (P less than 0.02) lower in the low K+ than in the control group (0.75 +/- 0.10 mumol/min/g placenta, n = 11, and 1.27 +/- 0.14 mumol/min/g placenta, n = 9, respectively). These results show that net placental K+ fluxes are unaltered during maternal hypokalaemia but suggest that unidirectional maternofetal fluxes may be reduced.