Substrate dependence and oxygen sensitivity of tone and of spontaneous and evoked contractions of the distal colonic muscularis mucosae of opossum.

The muscularis mucosae of the opossum distal colon fails to maintain tone and to generate spontaneous contractions in an anaerobic environment in vitro. To investigate this phenomenon further, dose-response curves to agonists under different oxygen concentrations were examined. In other experiments the tissue was actively exhausted (i.e., repeatedly stimulated in a substrate-free environment until no response to K+, acetylcholine, or histamine could be observed) and then equilibrated with an alternative substrate for 3 h, when the ability of the tissue to contract to agonists was reevaluated. Reducing the available oxygen abolished tone and caused a graded loss of responses to histamine; in contrast, at 0% oxygen maximum responses to acetylcholine were reduced by only 75%. Abolition of tone by the addition of cyanide to the bathing medium did not affect the amplitude of responses to histamine and acetylcholine. The tissue was able to use all the chosen alternative substrates to different degrees for acetylcholine-induced contraction; similarly, with the exception of beta-hydroxybutyrate it could use all substrates to some degree to support responses to histamine. The evidence suggests that the source of energy required to maintain tone is different from the source required to sustain K+ or agonist-induced contractions, and that the dependence of isotonic contractions on exogenous substrates varies with the pharmacologic stimulus.