Dual effect of phosphatase inhibitors on calcium channels in intestinal smooth muscle cells
Obara, K.; Yabu, H.
American Journal of Physiology 264(2 Pt 1): C296-C301
1993
ISSN/ISBN: 0002-9513 PMID: 8383427 Document Number: 417971
The effects of okadaic acid (OA) and calyculin A (CL-A), potent inhibitors of protein phosphatases type 1 (PP1) and type 2A (PP2A), on inward current carried by Ba-2+ through voltage-dependent Ca-2+ channel in guinea pig teniae coli smooth muscle cells were investigated using whole-cell patch-clamp technique. High concentrations of OA (5 times 10-8-5 times 10-6 M) and CL-A (10-9-10-7 M) dose dependently increased the inward current. The concentration producing apparent half-maximum enhancing effect values of OA and CL-A were 1.12 times 10-7 and 1.78 times 10-9 M, respectively. CL-A appeared to be apprx 100-fold more potent in increasing the inward current than OA. Lower concentrations of OA (10-10-2 times 10-8 M) and CL-A (10-11-10-9 M) decreased the inward current. The maximum inhibitory effects of OA and CL-A were observed at 10-8 M OA and 5 times 10-10 M CL-A, respectively. CL-A is apprx 100 times more effective inhibitor of PP1 than OA, and lower concentrations of OA and CL-A used in the present study inhibit PP2A activity, but they have no or little effect on PP1 activity (Ishihara, H., B. L. Martin, D. L. Brautigan, H. Karaki, H. Ozaki, Y. Kato, N. Fusetani, S. Watabe, K. Hashimoto, D. Uemura and D. J. Hartshorne. Biochem. Biophys. Res. Commun. 159: 871-877, 1989). In the absence of ATP in pipette solution, OA and CL-A did not affect the inward current. These results suggest that the increasing effects of higher concentrations of OA and CL-A on the inward current may involve PP1 activity and the decreasing effects of lower concentrations of them may involve PP2A activity, and that the effects of OA and CL-A may be dependent on the balance between PP1 and PP2A activities.