Phenylephrine induces relaxation of longitudinal strips from small arteries of goat legs

Abstract

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. Phenyl ephrine relaxes longitudinal strips of small arteries PLOS ONE | https://doi.org/10.1371/journal.pone.0227316 March 3, 2020 2 / 17 PE induces relaxation of longitudinal smooth muscle of small artery both under normal and high NO environmentsPE 100μmol/L reduced the tension of longitudinal smooth muscle from 0.25g to 0.09g in small arteries. (Median, n = 5, P value = 0.043, WSR), Fig 5, (i)). While L-Arginine 100μmol/L did not reduce the basal tension by itself, subsequent addition of PE 100μmol/L reduced the tension from 0.28 to 0.07g (Median, n = 5, P value = 0.043, WSR), Fig 5, (ii). Fig 4. (A) Raw tracing showing changes in vascular tension with PE 100μmol/L in transverse cylinder preparation of small artery. (B) Scatter plots of results from all five experiments demonstrating change in vascular tension with PE from baseline. ( � p = 0.043, WSR), (i) PE 100μmol/L (ii) PE in the presence of L-Arginine (iii) PE in the presence of NO synthase blocker L-NNA.

Enhanced Vasoconstriction Mediated by α1-Adrenergic Mechanisms in Small Femoral Arteries in Newborn Llama and Sheep Gestated at Low and High Altitudes

“…A similar pattern was described in the bronchial resistance artery in rabbits (Zschauer et al, 1997 ) and mesenteric resistances artery in rats (Dora et al, 2000 ). The proposed mechanism suggests that PHE promotes α 1 -adrenoceptor activation in endothelium, increasing NO production that opposes vasoconstriction (Tuttle and Falcone, 2001 ; Raj and Subramani, 2016 ; Marconi et al, 2020 ). This result suggests different subtypes of α 1 -adrenoceptor with a specific location in the small femoral artery, one type of α 1 -adrenoceptor present in the endothelium, and other α 1 -adrenoceptors present in smooth muscle cells, as proposed by Moraga et al ( 2011 ).…”

Electrical modelling of tissue experiments confirms precise locations of resistance and compliance in systemic arterial tree—they are mutually exclusive

“…This evidence must be seen in the context of the fact that the aorta has both longitudinal and circular smooth muscle and adrenergic stimulation causes constriction of both types of muscle. 13,19 On the other hand, there was no change in mean aortic pressure for the entire range of reduction in compliance, but the pulse pressure increased with reducing compliance almost linearly.…”

“…[10][11][12] However, the aorta has a considerable smooth muscle component in the tunica media, interspersed with which the elastic tissue is found. 13 The role of smooth muscle in the walls of the aorta and large arteries is less appreciated. 14 Vasoconstriction of the aorta should lead to an acute reduction in its compliance, and this component of compliance loss must be reversible with vasodilator drugs.…”

“…15,16 The aortic smooth muscles, both circular and longitudinal, contract in response to adrenergic stimulation, whereas in the arterioles, the circular muscles contract, and longitudinal muscles relax. 13 It is not clear, however, if vasoconstriction in the aorta also leads to a reduction in lumen diameter (i.e. increase aortic resistance).…”

“…17,18 Similarly, in small arteries, there is a considerable amount of elastic tissue along with the smooth muscles. 13 It is still not clear if the compliance, because of these elastic fibres, also drops during small artery vasoconstriction. One aim of this study is to assess the effect of adrenaline, a known vasoconstrictor, on aortic resistance and compliance when compared to that of small arteries.…”