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UBC Theses and Dissertations

Studies on the role of cyclic GMP in the regulation of contractility in heart and blood vessels Ng, David Dean Wing


This thesis is mainly concerned with the study of the role of cGMP in regulating contractility in the heart and blood vessels. A novel cGMP lowering agent, LY83583 (6-anilino-5,8-quinolinedione), was employed as a tool to determine the involvement of cGMP in mediating pharmacological and biological responses in the tissues being examined. In the first study, the role of cGMP in atriopeptin II-induced vascular relaxation was investigated. Atriopeptin II is believed to produce its vasorelaxant effect by virtue of its ability to elevate cGMP. However, the ability of the guanylate cyclase inhibitor, methylene blue, to inhibit the atriopeptin II-induced vasorelaxation has not been conclusively demonstrated. In the present study, LY83583 was found to partially prevent the rise in cGMP level caused by atriopeptin II but was without effect on the extent of the relaxation. This lack of correlation between cGMP elevation and relaxation may indicate either functional compartmentalization of the cyclic nucleotide or the existence of a cGMP-independent pathway for relaxation. Alternatively, the attenuated cGMP level may still be sufficient to elicit full relaxation. The inability of atriopeptin II to relax KC1-contracted bovine coronary arteries agrees with other reports in the literature and may indicate that the drug is less effective in antagonizing vascular responses associated with a marked degree of cell membrane depolarization. In the second study, the role of cGMP in mediating the endothelium-dependent inhibition of contractile responses of vascular tissue to alpha adrenoceptor stimulation was examined. There are reports in the literature that EDRF released from the endothelium elevates cGMP and depresses the response of the vessels to vasoconstrictors such as clonidine and norepinephrine. In the present study, LY83583 was used to examine the role of cGMP in mediating this effect. The treatment with LY83583 significantly lowered basal levels of cGMP and markedly enhanced the contractile response of endothelium-containing rat arteries to clonidine and norepinephrine. cGMP measurements indicate that clonidine did not elevate cGMP levels; hence the drug is unlikely to stimulate EDRF release. On the other hand, the depressant action of LY83583 on basal cGMP levels supports the hypothesis that inhibition of contractile responses may be a result of spontaneous release of EDRF, which results in tonic elevation of cGMP. The ability of 8-bromo-cGMP to reverse LY83583-induced enhancement of contractile responses to clonidine and norepinephrine further supports the involvement of cGMP in EDRF-induced vascular relaxation. In the final study, the role of cGMP in regulating cardiac contractility of amphibian ventricles was examined. The importance of cGMP in controlling mammalian cardiac function is controversial. However, a remarkable correlation between cGMP and contractile force has been reported in hypodynamic frog ventricles, and cAMP and cGMP were reported to act in a reciprocal fashion to regulate contractility. The present investigation attempted to verify whether such a relationship actually exists in the frog ventricles. Carbachol elicited a dose-dependent reduction in contractility without altering cGMP levels. On the contrary, sodium nitroprusside (100µM) did not reduce cardiac contractility despite a significant elevation of cGMP. At 1mM sodium nitroprusside, a huge elevation of cGMP and a small reduction in contractile tension were observed. Qualitatively similar results were obtained with a degraded sample of sodium nitroprusside. cAMP/cGMP ratios were not correlated with contractility. Hence, these findings were inconsistent with those found in earlier studies on hypodynamic frog hearts and do not support the proposed role of cGMP as a second messenger. The disparate findings may be caused by differences in experimental design. Alternatively, functional compartmentalization of cGMP (in the case of sodium nitroprusside) and the involvement of other cGMP-independent pathways (in the case of carbachol) cannot be ruled out. All these findings suggest that cGMP may play a more crucial role in regulating vascular than cardiac contractility.

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