A comparative study of endothelin and sarafotoxin action in vascular and non-vascular smooth muscle

Eta, Ekong Ito Ekong (1991) A comparative study of endothelin and sarafotoxin action in vascular and non-vascular smooth muscle. Masters thesis, Memorial University of Newfoundland.

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Abstract

The hypothesis that endothelin (ET-1) is the endogenous ligand for “sarafotoxin receptors” (SRTX receptors) was examined. The actions of ET-1 and SRTX-b were compared with those of the well characterized vasoconstrictor, norepinephrine (NE). The rationale was to identify and compare the transduction-effector mechanisms of these peptides versus NE in vascular and non-vascular smooth muscles. – In this study, the rat aorta and anococcygeus muscles were used as representatives of vascular and non-vascular smooth muscle, respectively. The role of extracellular Ca²⁺ was studied by omitting Ca²⁺ from the physiological saline solution, and the contribution of voltage-operated Ca²⁺ channels (VOCCs) assessed by determining the effects of nifedipine. In addition, the effects of ryanodine, which interferes with the release of cellular Ca²⁺, was also studied. The roles of arachidonic acid products were determined by studying the effects of the cyclooxygenase inhibitor, indomethacin, and the lipoxygenase inhibitor, nordihydroguaiaretic acid (NDGA). The contractions elicited by ET-1 and SRTX-b (10 nM), or NE (1μM) were approximately equieffective in terms of tension development and correspond to EC₅₀ values, and these concentrations were thus used throughout the study. In Ca²⁺-free Krebs, the three agonists generated approximately similar levels of tone in the aorta and the anococcygeus corresponding to 18 and 5% of the maximum response, respectively. Nifedipine, 10 μM, significantly inhibited responses to ET-1 in both the aorta and anococcygeus; the responses to SRTX-b and NE were, however, not significantly affected in either tissue. A combination of 10 μM ryanodine and nifedipine caused near complete inhibition of response to ET-1 in the aorta and also significantly reduced the response to both ET-1 and NE in the anococcygeus. The lipoxygenase inhibitor, NDGA, inhibited the response to ET-1 in the aorta and ET-1 and NE in the anococcygeus muscle. The cyclooxygenase inhibitor, indomethacin, however, had no effect on the responses to any of the three agonists in either the aorta or anococcygeus. At concentrations greater than 30 nM both ET-1 and SRTX-b induced myogenic activity in normally quiescent anococcygeus muscle. As determined by the loss of myogenic activity, the tissues recovered more rapidly from SRTX-b than ET-1, with complete recovery apparent after 2.62 ± 0.85 and 5.22 ± 0.06 h, respectively. Omitting Ca²⁺ from the Krebs solution reduced recovery times to 1.62 ± 0.2 and 2.4 ± 0.51 h, respectively. -- In conclusion, the studies performed indicate that a number of cell signaling processes are activated by ET-1 and SRTX-b in smooth muscle and this could account for the varied responses. The suggestion and ET-1 solely acts on the ‘SRTX receptor,’ as proposed by Kloog and Sokolovsky (1989), is probably too simplistic. Results from this study also indicate that tissue variability does exist and, indeed, subclasses of ET/SRTX receptors have been inferred by others (Yanagisawa and Masaki, 1989b; Nayler, 1990).

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