Size‐dependent heterogeneity of contractile Ca2+ sensitization in rat arterial smooth muscle
Authors: Kitazawa Toshio , Kitazawa Kazuyo 
Published: August 2012

Key points
Each segment along arterial vessels adapts to different circumstances, such as high blood pressure in the central and low pressure in the peripheral arteries and high sympathetic innervation in the peripheral and low innervation in the central arteries.
We tested, using pharmacological tools, whether the amplitude and time course of each signalling pathway varies dynamically between arterial segments in rat.
In small mesenteric arteries, α1‐agonist produced a contraction and myosin light chain phosphorylation through sequential activation of α1A‐subtype receptors, Ca2+, PKC and protein kinase C‐potentiated protein phosphatase inhibitor protein 17 kDa (CPI‐17).
In large aorta, α1‐agonist‐induced contraction and phosphorylation were produced through activation of α1D receptors followed by a Ca2+ increase and constitutively active Rho‐kinase in an independent manner. The results for midsized arteries were intermediate.
Our findings provide insights into the development of new therapeutic agents controlling the size‐dependent vasoconstriction.
Abstract  Each segment along arterial vessels adapts to different circumstances, including blood pressure and sympathetic innervation. PKC and Rho‐associated kinase (ROCK) Ca2+‐sensitizing pathways leading to myosin phosphatase inhibition are critically involved in α1‐adrenoceptor‐mediated vascular smooth muscle contraction in distinctive time‐dependent manners. We tested whether the amplitude and time course of each pathway varies dynamically between arterial segments. Using pharmacological approaches, we determined the time‐dependent roles of Ca2+ release, Ca2+ influx, PKC and ROCK in α1‐agonist‐induced contraction and phosphorylation of key proteins in denuded rat small mesenteric artery, midsized caudal artery and thoracic aorta. SR Ca2+ release and voltage‐dependent Ca2+ influx were essential for the initial rising and late sustained phases, respectively, of phenylephrine‐induced contraction, regardless of arterial size. In small mesenteric arteries, α1A‐subtype‐specific antagonists and inhibitors of PKC, but not ROCK, markedly reduced the initial and late phases of contraction in a non‐additive manner and suppressed phosphorylation of myosin light chain (MLC) and CPI‐17, but not myosin targeting subunit of myosin light chain phosphatase (MYPT1). In aorta, an α1D‐specific antagonist reduced both the initial and late phases of contraction with a significant decrease in MLC but not CPI‐17 or MYPT1 phosphorylation. ROCK inhibitors, but not PKC inhibitors, suppressed the sustained phase of contraction with a decrease in MLC and MYPT1 phosphorylation in the aorta. The effect of ROCK inhibitors was additive with the α1D‐antagonist. The results for midsized arteries were intermediate. Thus, the PKC–CPI‐17 Ca2+‐sensitizing pathway, which is dependent on PKC subtype and a Ca2+‐handling mechanism, and is downstream of α1A receptors, plays a major role in α1‐agonist‐induced contraction of small resistance arteries in the splanchnic vascular beds. The effect of PKC and ROCK increases and decreases, respectively, with decreasing arterial size.