The effects of protein kinase C on trabecular meshwork and ciliary muscle contractility

PURPOSE. The possible role of protein kinase C (PKC) inhibitors in novel pr essure-lowering drugs is currently under investigation. To gain further ins ight into regulation of contractility by PKC in trabecular meshwork (TM) an d ciliary muscle (CM), the effects of various PKC inhibitors and activators were tested. METHODS. Isometric tension measurements of bovine TM and CM strips were per formed. PKC was stimulated by phorbol ester and by the diacylglycerol analo gue diC(8). PKC blockade was accomplished using H7 and myristoilated PKC su bstrate (mPKC). Western blot analysis was used to identify specific PKC iso forms in human trabecular meshwork (HTM), human ciliary muscle (HCM), and b ovine TM and CM. RESULTS. In tissues precontracted by carbachol PKC antagonist H7 led to a r elaxation of TM (25 +/- 7.2 versus 100%; n = 8) with no effect on CM. mPKC substrate selectively blocks PKC. This substance led to relaxation of TM (3 2.8 +/- 7.4 versus 100%, n = 7), whereas CM was not affected. PMA at concen trations of 10(-6) M led to a slow contraction of both tissues that was mor e marked in TM. DiC(8) and 4 alpha-phorbol had no effect on contractility. Western blot analysis revealed expression of calcium-dependent PKC-alpha an d calcium-independent PKC-epsilon isoforms in HTM and HCM. PKC-epsilon expr ession was more pronounced in HTM than in HCM. Similar PKC isoform expressi on was found in native bovine tissue. CONCLUSIONS. PKC isoforms show different tissue distributions in human and bovine TM and CM. Contractility differences exist in both tissues in respon se to PKC antagonists and agonists. The data indicate that PKC may be invol ved in regulation of aqueous humor outflow by the TM. Thus, inhibition of P KC may represent a new way of influencing outflow facility through isolated relaxation of TM.