Effects of cholecystokinin octapeptide on the contractile activity of guinea-pig colonic smooth muscles, L-type calcium currents and membrane potentials of myocytes

Objective: To investigate the effects and mechanism of cholecystokinin octapeptide (CCK-8S) on the contractile activity of smooth muscles, L-type calcium current and membrane potentials of proximal colon myocytes in guinea pig.

Methods: (1) Strips of proximal colon were obtained from adult guinea pigs. The contraction of these stripes was measured by a RM6240 multi-channel physiological signal system. (2) Suspension of single smooth muscle cells (SMCs) were obtained from proximal colon and isolated by enzymatic digestion. The effect of CCK-8S on intracellular calcium concentration ([Ca(2+)]i) of SMCs was examined by fura-2-loaded microfluorimetric measurement. (3) Resting potential (RP), action potential (AP) and L-type calcium current (I(Ca-L)) were recorded by patch-clamp technique.

Results: (1) The contractile amplitude and frequency of muscle stripes enhanced by CCK-8S (10(-7) mol/L) were (149 ± 12)% and (132 ± 13)% respectively of those of control group (all P < 0.05). They were significantly attenuated by pretreating strips with CCK1 receptor antagonist devazepide (10(-7) mol/L), L-type calcium channel blocker nifedipine (10(-5) mol/L), Ca(2+)-ATPase inhibitor TG (thapsigargin) (10(-5) mol/L) and BA (boric acid) (10(-5) mol/L) respectively. (2) [Ca(2+)]i of SMCs intensified by CCK-8S was (738 ± 24)% of that of control group. And it was inhibited by pretreating SMCs with devazepide (all P < 0.05). (3) After the superfusion of CCK-8S, RP depolarized to (52 ± 9)%, the exogenously stimulated peak values of AP rose to (140 ± 4)% and fast repolarization time of AP decreased to (61 ± 13)% (all P < 0.05). They were significantly inhibited when these cells were pretreated with devazepide and/or nifedipine (n = 8, P < 0.05 for each group) whereas CI 988 had little effect. (4) The CCK-8S-evoked I(Ca-L) of SMCs at the voltage of + 10 mV was boosted to (138 ± 7)%. Such an effect was suppressed by a pretreatment with nifedipine, devazepide, TG and BA respectively. In the presence of an inhibitor of inositol 1,4,5-trisphosphate (IP3) receptors, heparin (10(-6) mol/L) and an protein kinase C (PKC) inhibitor, saurosporine (10(-6) mol/L), CCK-8S did not significantly intensify I(Ca-L) (all P > 0.05).

Conclusions: CCK-8S promotes proximal colon contraction by CCK1 receptors through the activation of IP3-mediated PKC pathway.