Evidence that tolerance and dependence of guinea pig myenteric neurons to opioids is a function of altered electrogenic sodium-potassium pumping.

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 microM), 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 microM ouabain was eliminated, and the proportion of cells depolarized by ouabain increased from 31% to 83%. Glibenclamide (10 microM) and 100 nM iberiotoxin did not change the pattern of membrane potential changes induced by 1 microM ouabain. Calcium-free buffer eliminated the hyperpolarization and potentiated the depolarization induced by 1 microM ouabain. Ouabain (5 microM), 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 microM), 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.