Effects of the beta‐adrenergic agonist, ritodrine, and insulin on plasma potassium concentrations in fetal lambs

Prolonged infusion of the beta 2‐adrenergic agonist, ritodrine, into sheep during late pregnancy decreased maternal plasma K+ from 3.6 to 2.5 mmol l‐1 during the first 6‐8 h of infusion, as it does during tocolysis in women. This decrease was not accompanied by significant change in fetal plasma K+ concentration. Ritodrine infusion (1‐3.5 micrograms kg‐1 min‐1) directly into the fetus also did not decrease fetal plasma K+ significantly. In contrast, insulin (2.5 mU kg‐1 min‐1), infused together with glucose (3.6 mg kg‐1 min‐1) directly into the fetus decreased fetal plasma K+ concentration by 0.8 mmol l‐1 within 1 h. The results suggest immaturity in beta 2‐adrenergic receptor regulation of electrogenic K+ uptake by muscle in fetal lambs.