Correlation of Contractile Function of the Rabbit Corpus Cavernosum with NADH Fluorescence

The NADH/NAD ratio is a measure of potential metabolic energy in smooth muscle tissue. Previous studies on bladder smooth muscle demonstrated that during active contraction when energy utilization is high, NADH is rapidly oxidized to NAD resulting in a decrease in the ratio of NADH/NAD. Intracellular systems utilizing ATP as an energy source are characterized by changes in the NADH/NAD ratio. This ratio can be monitored simultaneously with changes in smooth muscle tissue tone using an optical fiber probe which continually monitors NADH fluorescence and contractile activity of the same smooth muscle preparation. The present study correlates alterations in the ratio of NADH/ NAD with both spontaneous and induced contractions and relaxations in the rabbit corpora cavernosa. The results show a high degree of correlation between a decrease in spontaneous fluorescence (decrease in the NADH/NAD ratio) and spontaneous contraction. An increase in tension was followed in time by a decrease in the NADH/NAD ratio. This was consistent for all strips showing significant spontaneous activity. ATP caused a rapid decrease in tension which was correlated with a decrease in fluorescence. The relative decrease in NADH fluorescence was proportional to the relative decrease in tension. Under basal conditions (0.8 g passive tension) ATP and nitro-prusside stimulated a marked reduction in tension, but only ATP stimulated a substantial decrease in NADH fluorescence. Bethanechol and isoproterenol relaxed the corporal tissue to a relatively small degree which correlated with relatively small decreases in fluorescence. Methoxamine stimulated a substantial contraction of corporal smooth muscle which correlated with a rapid and significant decrease in NADH fluorescence. Following precontraction by methoxamine, ATP stimulated a marked decrease in both tension and fluorescence. Nitroprusside and bethanechol similarly stimulated marked decreases in tension with only minor changes in NADH fluorescence. Isoproterenol induced a minor decrease in tension with no significant change in NADH fluorescence from that of the basal response. The dissociation of the endothelium from the corporal tissue inhibited the response to bethanechol by 90% for both fluorescence and tension while the response to ATP was virtually unaffected. In summary, the data indicate a strong correlation between stimulation of contraction by methoxamine and a decrease in the NADH/NAD ratio; and strong relaxation of corporal smooth muscle by ATP and a decrease in the NADH/NAD ratio. Since these drugs have opposite effects on tension, this result may indicate that both contraction- and ATP-induced relaxation in the rabbit corpus cavernosa are active processes and depend on the rapid utilization of cellular metabolic energy. It appears that corporal relaxation mediated by nitroprusside is highly efficient, utilizing significantly less intracellular energy and producing maximal relaxations.