Calcium dependence of indolactam-mediated contractions in resistance vessels.
The protein kinase C activator, (-)-indolactam, has been shown to enhance reactivity of arterioles by a mechanism not requiring an increase in intracellular Ca++ (Ca++i). The aim of this study was to characterize the Ca++ requirement for indolactam-induced contraction of resistance vessels. Studies were performed in small mesenteric arteries (diameter, 260 +/- microm), using intact segments or after permeabilization with alpha-toxin (500 U/ml). Additional studies were preformed using isolated smooth muscle cells to allow electrophysiological assessment of the effect of indolactam on voltage-gated Ca++ entry. Intact and permeabilized vessel segments showed dose-dependent constriction to indolactam. Studies of Fura 2-loaded vessels and permeabilized segments maintained at low Ca++i, showed that the constriction occurred without an overt increase in Ca++i. That Ca++ was required was evident by near maximal relaxation after the removal of Ca++. Patch clamp studies indicated that indolactam potentiated voltage-gated Ca++ entry; however, nifedipine (0.5 microM) and La (0.2 or 1 mM) were relatively ineffective in reversing the contraction, indicating that voltage-gated Ca++ entry was not an absolute requirement. In intact vessel segments, the myosin light chain (MLC) kinase inhibitors, ML-7 and ML-9, reversed the indolactam contraction, suggesting the requirement of the MLC chain phosphorylation pathway. Furthermore, indolactam caused by a significant increase in MLC phosphorylation in permeabilized vessels, despite clamping of Ca++i at pCa 7.0. The data are consistent with the suggestion that the protein kinase C activator, indolactam, acts to modulate the Ca++ sensitivity of the smooth muscle contractile process such that higher than expected levels of MLC phosphorylation exist for a given level of Ca++i.
Hill
M A
MA
Department of Physiology, Eastern Virginia Medical School, Norfolk, USA.
Davis
M J
MJ
Song
J
J
Zou
H
H
D000818:Animals; D001808:Blood Vessels / Q000187:drug effects*; D002118:Calcium / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D004594:Electrophysiology; D007211:Indoles / Q000494:pharmacology*; D007769:Lactams / Q000494:pharmacology*; D008297:Male; D008643:Mesentery / Q000187:drug effects; D009119:Muscle Contraction / Q000187:drug effects*; D009638:Norepinephrine / Q000494:pharmacology; D011493:Protein Kinase C / Q000187:drug effects*; D051381:Rats; D017207:Rats, Sprague-Dawley; D013997:Time Factors
D007211:Indoles; D007769:Lactams; C043969:indolactam V; D011493:Protein Kinase C; D002118:Calcium; D009638:Norepinephrine