Background: 5-hydroxytryptamine (5-HT) is released into the intestinal lumen during the fasting state. However, the relationship between the intraduodenal 5-HT and the interdigestive cyclic motor activity in conscious dogs is unclear. Aim: To correlate intraduodenal 5-HT concentrations with the interdigestive gastroduodenal migrating motor complex (MMC). Methods: 6 dogs were implanted with 2 force transducers for recording gastroduodenal contractions and 2 catheters for measuring duodenal volume by a non-absorbable marker perfusion technique. Intraduodenal 5-HT concentrations were determined by high performance liquid chromatography at 5-min intervals. Results: During fasting, gastroduodenal motor activity cycled as the MMC; luminal 5-HT concentrations and total outputs varied cyclically in temporal association with the MMC. Mean 5-HT concentrations peaked during phase II (P<0.05 vs. phase I and III), and 5-HT outputs during phases II or III were greater than during phase I (P<0.05). Exogenous motilin (0.3 μg/kg-hr, IV) stimulated 5-HT release into the duodenal lumen with peak values (P<0.05) during motilin-induced phase II and III. Gastroduodenal motor activity was not altered, however, during exogenous intraduodenal administration of 5-HT (300 ng/mL-min). Conclusions: 5-HT is released cyclically into the duodenal lumen in close temporal association with the MMC, but its physiologic significance in regulation of gastroduodenal motility is unknown.

View full abstract

We investigated which isoform of protein kinase C (PKC) is responsible to metabolic inhibition in the guinea pig taenia caeci with respect to their phosphorylation levels. By Western blot analysis using isoform-specific antibodies, at least four isoforms of PKC, α, β2, ε and ζ were identified in the taenia. Prolonged metabolic inhibition of hypoxia, hypoxia+glucose depletion, and addition of cyanide (all in the presence of high K⁺) for more than 60 min, but not glucose-depletion only, elicited dephosphorylation of PKCs, α, β2 and ε, except ζ. Ca²⁺ depletion from the medium prevented the dephosphorylation of PKCs induced by hypoxia, and apparently inhibited the dephosphorylation induced by hypoxia+glucose depletion. Acute treatment with hypoxia for 10-30 min elicited a gradual dephosphorylation of PKCβ2, but not of other tested PKC isoforms. Considering the ATP level under various metabolic conditions reported previously, PKCβ2 is suggested to be primarily responsible to hypoxia, and its dephosphorylation is closely associated with the alteration of adenylate compounds in the cell. Re-oxygenation after prolonged hypoxia did not restore the phosphoryation level of any tested PKCs, suggesting that the dephosphorylation of PKCs is associated with the irreversible damage of the cell under hypoxia. Presumably, the dephosphorylaton of PKCs, particularly PKCβ2, plays a role in the signal transduction pathway under metabolic inhibition of the taenia, as reported in proliferative and pathophysiological processes in many other cells.

View full abstract

MaxiK channel, the large-conductance Ca²⁺-sensitive K⁺ channel, facilitates a negative feedback mechanism to oppose excitation and contraction in various types of smooth muscles including urinary bladder smooth muscle (UBSM). In this study, we investigated how the contribution of MaxiK channel to the regulation of basal UBSM mechanical activity is altered in streptozotocin-induced diabetic rats. Although the urinary bladder preparations from both control and diabetic rats were almost quiescent in their basal mechanical activities, they generated spontaneous rhythmic contractions in response to a MaxiK channel blocker, iberiotoxin (IbTx). The effect of IbTx on the mechanical activity was significantly greater in diabetic rat than in control animal. Similarly, the basal mechanical activity was increased with apamin, an inhibitor for some types of small conductance Ca²⁺-sensitive K⁺ channels, and this effect was more pronounced for diabetic rat. However, in both control and diabetic animals, IbTx action was stronger than that of apamin. Diabetes also enhanced the responses to BayK 8644, an L-type Ca²⁺ channel agonist. The extent of this enhancement in diabetic bladder vs. control was, however, almost the same as that attained with IbTx. Expression levels for MaxiK channel as well as apamin-sensitive K⁺ channels and L-type Ca²⁺ channel were not altered by diabetes, when determined as their corresponding mRNA levels. These results indicate that diabetes can potentially increase the basal UBSM mechanical activity. However, in diabetic UBSM, the main negative-feedback system triggered by MaxiK channel is still preserved enough to counteract the possible enhancement of this smooth muscle mechanical activity.

View full abstract

We have investigated the effects of cilostazol, a type III phosphodiesterase inhibitor, on the electrical responses of smooth muscle tissue isolated from the guinea-pig stomach antrum. Cilostazol (10^-5 M) inhibited slow waves recorded from circular muscle cells, but did not significantly alter the pacemaker potentials and follower potentials recorded from myenteric interstitial cells and longitudinal muscle cells respectively. Slow potentials generated in isolated circular muscle bundles without attached myenteric interstitial cells were inhibited by cilostazol (>10^-7 M), while all membrane activities were abolished by 10^-5 M cilostazol. In circular muscle bundles, the input resistance of smooth muscle cells and the refractory period for the generation of slow potentials were not altered during the inhibition of spontaneous activity with cilostazol. While cilostazol at 10^-7 and 10^-6 M did not elevate the tissue content of cyclic AMP, at 10^-5 M cyclic AMP was elevated by about 30%. A similar elevation was also produced by 10^-7 M forskolin. The content of cyclic AMP was not significantly increased in preparations stimulated with 10^-3 M caffeine. The potency for inhibiting slow waves was in the order caffeine (10^-3 M) > forskolin (10^-7 M) > cilostazol (10^-5 M). The frequency of slow waves was decreased by caffeine or forskolin but not by cilostazol, while the duration was reduced by caffeine but not by cilostazol or forskolin. Follower potentials were modulated by caffeine and forskolin, but not by cilostazol: the duration was reduced by caffeine, the frequency was reduced by caffeine or forskolin, and the amplitude was not significantly altered by any of them. The results indicate that cilostazol has high selectivity in inhibiting the activity of circular muscle much more than that of longitudinal muscle or pacemaker cells, with no causal relation to the tissue content of cyclic AMP as appears to be the case for the inhibitory actions of caffeine and forskolin.

View full abstract