Vol. 280, Issue 2, 593-599, 1997

Evidence that Tolerance and Dependence of Guinea Pig Myenteric Neurons to Opioids Is a Function of Altered Electrogenic Sodium-Potassium Pumping¹

Jian-Qiang Kong, Judith A. Leedham, David A. Taylor and William W. Fleming

Department of Pharmacology and Toxicology, West Virginia University, Robert C. Byrd Health Sciences Center, Morgantown, West Virginia

Ouabain acutely depolarizes most types of cells through inhibition of electrogenic Na⁺,K⁺ pumping and is a useful tool with which to study conditions that affect electrogenic pumping. Intracellular recording techniques were used with neurons of the guinea pig myenteric plexus/longitudinal muscle preparation exposed to ouabain. Of 35 S neurons exposed to ouabain (1 µM), 15 were hyperpolarized by 10 ± 2 mV, 11 were depolarized by 8 ± 2 mV and the remaining neurons had no change in membrane potential. The nonselective potassium channel antagonist tetraethylammonium chloride (TEA; 0.5 mM) alone evoked modest (<5 mV) and inconsistent changes in the resting membrane potential of S neurons. However, in the presence of TEA, the hyperpolarizing response to 1 µM ouabain was eliminated, and the proportion of cells depolarized by ouabain increased from 31% to 83%. Glibenclamide (10 µM) and 100 nM iberiotoxin did not change the pattern of membrane potential changes induced by 1 µM ouabain. Calcium-free buffer eliminated the hyperpolarization and potentiated the depolarization induced by 1 µM ouabain. Ouabain (5 µM), in either the presence or absence of TEA, induced depolarization in all neurons tested (mean, 15-16 mV), indicating a predominant effect of inhibition of electrogenic pumping. These data suggest that ouabain may directly or indirectly activate myenteric S neuron calcium-sensitive potassium channels as well as inhibit the Na⁺,K⁺ pump and that TEA will antagonize the former effect. Chronic exposure (morphine pellets) of guinea pigs to morphine resulted in a partial depolarized state of myenteric neurons, as previously reported. Ouabain (5 µM), either with or without TEA, depolarized neurons from chronically morphine-treated guinea pigs very little (5-6 mV) in comparison with naive neurons (15-16 mV). This supports the conclusion that the depolarized state of morphine-tolerant neurons is associated with a reduction in electrogenic Na⁺,K⁺ pumping.