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p38^MAPK Inhibition Enhances Basal and Norepinephrine-Stimulated p42/44^MAPK Phosphorylation in Rat Pinealocytes¹

Department of Physiology (M.M., J.R.M., A.K.H.) and Department of Medicine (C.L.C.), Faculty of Medicine, University of Alberta, 7-26 Medical Sciences Building, Edmonton, Alberta T6G 2H7, Canada

Address all correspondence and requests for reprints to: Dr. A. K. Ho, Department of Physiology, 7–26 Medical Sciences Building, Edmonton, Alberta T6G 2H7, Canada. E-mail: anho{at}ualberta.ca

Interaction between p38^MAPK and p42/44^MAPK in rat pinealocytes was examined by determining the effects of p38^MAPK inhibitors on the phosphorylation of p42/44^MAPK using Western blot analysis. Treatment with SB202190, a specific inhibitor of p38^MAPK, increased p42/44^MAPK phosphorylation in a concentration-dependent manner. SB202190 also enhanced the magnitude and the duration of norepinephrine-activated p42/44^MAPK phosphorylation. The effect of SB202190 on p42/44^MAPK phosphorylation was abolished by PD98059 or UO126, inhibitors of MEK, suggesting that SB202190 is acting upstream of MEK in activating p42/44^MAPK. The SB202190-induced phosphorylation of p42/44^MAPK was not blocked by inhibitors of cGMP-dependent kinase (KT5823), protein kinase C (calphostin C) or Ca²⁺/calmodulin dependent kinase (KN93) suggesting that these pathways may not be involved in the effect of SB202190. SB202190 further increased p42/44^MAPK phosphorylation that was stimulated by 8-bromo-cGMP, 4ß phorbol 12-myristate 13-acetate, or ionomycin. In contrast, inhibition of p42/44^MAPK phosphorylation by dibutyryl-cAMP persisted when p42/44^MAPK phosphorylation was increased by SB202190. Furthermore, inhibition of p42/44^MAPK phosphorylation had no effect on p38^MAPK activation. These results suggest that inhibition of p38^MAPK causes activation of p42/44^MAPK and acts synergistically with norepinephrine in the regulation of p42/44^MAPK activation in rat pinealocytes.

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