Regional differences in potassium content of smooth muscle from opossum esophagus

Strips from the proximal part of the smooth muscle segment of opossum esophagus have a significantly higher potassium content (50 +/- 3 meq/kg) than those from the distal part (38 +/- 3 meq/kg). There are no significant differences between the two regions in content of sodium (65 +/- 4 meq/kg in proximal, 71 +/- 3 meq/kg in distal) or chloride (48 +/- 10 meq/kg in proximal, 42 +/- 5 meq/ kg in distal). The mean [14C]inulin uptake is 240 +/- 10 ml/kg in both proximal and distal strips. [14C]polyethylene glycol uptake is smaller and [14C]sucrose and [14C]mannitol uptake in both areas are larger than that of inulin. Intracellular potassium concentration (based on the inulin uptake as an estimate of the extracellular space volume) is significantly higher proximally (71 +/- 3 mM) than distally (52 +/- mM). Ouabain, 10(-4) M, increases the intracellular concentration of sodium and decreases the intracellular concentration of potassium in both the proximal and distal segment. The efflux of 86Rb, measured by a washout technique, is higher in the distal than in the proximal smooth muscle segment. A difference in membrane permeability to rubidium and hence, potassium between proximal and distal smooth muscle segments may account in part for the different intracellular potassium concentrations.