UBC Theses and Dissertations
Studies on the role of cAMP and its activation of cAMP-dependent protein kinase in the mediation of vascular smooth muscle relaxation Zammit, Danielle Christine
The adenosine 3',5'-cyclic monophosphate (cAMP) activation of cAMP-dependent protein kinase (PKA), and its resultant effect on vascular smooth muscle tone was investigated. Isoproterenol (ISO) and prostaglandin E₁ ( PGE₁) have both been shown to elevate cAMP in vascular smooth muscle, the increase being accompanied by relaxation with ISO and contraction with P G E T . The activation of different pools of PKA by these drugs has been suggested to explain a similar observation in cardiomyocytes, where both ISO and PGE₁ elevated cAMP, yet only ISO increased contractility. To determine whether the circumstances in vascular smooth muscle were analogous to those in cardiac tissue, the study investigated the elevation of cAMP and the activation of PKA in subcellular fractions of vascular preparations treated with 1μM ISO and 10μM PGE₁. A 1μM forskolin (FSK)/ 10μM PGE₁ combination was used as a positive control for cAMP measurement. An integral component was the use of the rabbit aorta medial strip, consisting of only smooth muscle cells. It was not possible to observe PKA activation in any of these experiments. Studies in aortic strips denuded of the endothelium demonstrated a large increase in cAMP levels in response to the FSK/PGE₁ combination. ISO had no effect on cAMP levels in helical strips, although it relaxed phenylephrine-induced contractions. Contrary to observations in helical strips, no difference in cAMP levels was recorded in any cell fraction from the medial strips, with any of the drugs used. ISO relaxed medial strips, but cAMP assays present no evidence of an increase in cAMP being related to the relaxation of vascular smooth muscle. The results obtained are not sufficient to either support, or negate, the hypothesis that cAMP elevation and the subsequent activation of PKA plays an important role in vascular smooth muscle relaxation. The data suggest that a) it is possible to measure cAMP levels in different subcellular fractions of vascular smooth muscle and b) differences exist between the tunica media and the adventitia with respect to their functional and cAMP responses to these drugs. Further investigation of the functional compartmentation of cAMP and PKA in the cell, and the signalling systems in the different layers of the blood vessel wall may elucidate the role of the cAMP/PKA system in the control of vascular tone.
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