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The purpose of the present investigation was to compare calcium sensitivity of contractile machinery in aorta and portal vein smooth muscle cells (SMC) in normotensive Wistar—Kyoto (WKY) rats and spontaneously hypertensive Okamoto rats (SHR), and to shed light upon the mechanisms of possible differences.Investigations into calcium sensitivity of SMC myofilaments can only be made on skinned muscular strips.The vascular strips were made hyperpermeable by detergent skinning with saponin. The isometric calcium-induced contractions of SMC were recorded using a force displacement transducer coupled to a physiograph.It was shown that the pCa-tension (negative logarithm of calcium concentration versus tension) relationship for aorta and portal vein SMC in SHR shifted to the left in comparison with WKY rats. Putative protein kinase C inhibitors 1-(S-isoquionolinyl-sulfonyll)-2-methylpiperasine (H-7) and polymyxin B shifted the pCa-tension relationship more significantly to the right in the SMC of SHR than in WKY rats. It has also been shown that H-7 and polymyxin B sharply reduced the maximum tension developed by SMC in SHR whilst causing a non-significant decrease in maximum tension of SMC from WKY rats. These results are consistent with higher protein kinase C activity in SMC of SHR.These results indicate that the increase in calcium sensitivity of vascular SMC contractile machinery in SHR may be linked with the increase in their protein kinase C activity.