Key result
Phenylephrine induced relaxation of longitudinal smooth muscle strips from small arteries of goat legs, reducing tension from 0.25g to 0.09g, a response mediated by alpha-1D receptors.
Why the study?
Alpha adrenergic stimulation is known to produce vasoconstriction, but phenylephrine-induced vasorelaxation was previously observed in spiral strips of small arteries under high nitric oxide conditions.
This study demonstrates a novel phenomenon where alpha-adrenergic stimulation with phenylephrine induces relaxation in longitudinal smooth muscle of small arteries, mediated by alpha-1D receptors.
Answers are grounded in this paper and cite their sources.
Warrants caution extrapolating adrenergic effects from animal arteries; leaves open alpha-1D receptor role in human vascular tone.
Alpha adrenergic stimulation is known to produce vasoconstriction. We have earlier shown that, in spiral strips of small arteries Phenylephrine (PE) caused vasorelaxation under high nitric oxide (NO) environment. However on further experimentation it was realized that the PE-induced vasorelaxant response occurred only with longitudinal strips of small arteries even under normal NO environment while circular strips showed contraction with PE even under high NO environment. Such PE-induced vasorelaxation of longitudinal strips was blocked by Phentolamine, an alpha-adrenergic receptor blocker. On delineation of specific receptor subtype, PE-induced relaxation was found to be mediated through alpha 1D receptor. However, this phenomenon is specific to small artery, as longitudinal smooth muscle of aorta showed only contractile response to adrenergic stimulation. There is no prior report of longitudinal smooth muscle in small artery up to our knowledge. The results of this study and histological examination of vessel sections suggest the presence of longitudinal smooth muscle in small artery and their relaxant response to alpha adrenergic stimulation is a novel phenomenon.
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Marconi et al. (2019) studied Vascular smooth muscle physiology. Phenylephrine vs. Baseline tension was evaluated on Change in vascular tension in longitudinal strips of small arteries (p=0.043). Phenylephrine induced relaxation of longitudinal smooth muscle strips from small arteries of goat legs, reducing tension from 0.25g to 0.09g, a response mediated by alpha-1D receptors.
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