Proceedings of The Physiological Society
University of Edinburgh (2011) Proc Physiol Soc 25, PC41
Modulatory effects of K+ channel blockers on contractile actions of P2X1-purinoceptor agonists in human vas deferens
N. I. Amobi2, J. Guillebaud2, C. I. Smith1
1. King's College London, London, United Kingdom. 2. Elliot-Smith Clinic, Headington, Oxford, United Kingdom.
Contractions of human vas deferens circular muscle to nerve stimulation and noradrenaline are modulated by intermediate (IKCa) but not large (BKCa) conductance Ca2+-activated K+ channels (Medina et al., 2010). This finding raises issues on the roles of BKCa and delayed rectifier (KV) K+ channels identified in electrophysiological studies of smooth muscle cells isolated from human vas deferens (Park et al., 2004). The current study investigated the effects of K+ channel blockers on the contractile actions of α,β-methylene ATP (α,β-meATP) and ATP in muscle types of human vas deferens in order to clarify the role of K+ channels in modulating P2X1-receptors-induced contractility and excitability of the tissues. Human vas deferens specimens obtained after elective vasectomies (with patients’ consent and College Ethical approval) were cut into longitudinal muscle strips or rings of circular muscle. These were set up for tension recording and superfused with Krebs medium (36o C; pH 7.4). The effects of K+ channel blockers were determined on α,β-meATP-induced contractions. α,β-meATP (3-100 μM) evoked transient contractions of longitudinal but rarely of circular muscle. The contractions were blocked by suramin (300 μM). Both muscle types were unresponsive to ADP-βS (100 μM), a P2Y-receptor agonist. ATP (0.1-3 mM) activated only longitudinal muscle even in ARL 67156 (100 μM), an ecto-ATPase inhibitor. Longitudinal muscle contractility to α,β-meATP were enhanced by FPL 64176 (1 μM), an L-type Ca2+ agonist; 338-368%), TEA (0.3-1 mM), non-specific K+ channel blocker, 4-aminopyridine (0.1-0.3 μM), selective KV channel blocker and iberiotoxin (0.1 μM, P<0.01), selective BKCa channel blocker. Notably, quiescent circular muscles responded to α,β-meATP in FPL 64176 (1 μM) and robustly in iberiotoxin (0.1 μM). Apamin (0.1 μM), an SKCa channel blocker was ineffective in both muscle types. The results indicate that stimulation of P2X1-receptors in human vas deferens elicits excitatory effects that (a) lead to longitudinal muscle contraction and secondarily to activation of KV and BKCa channels and (b) is sub-contractile in circular muscle primarily due to ancillary activation of BKCa channels. The results provide an experimental basis for resolving not only conflicting reports on the involvement of purinergic activation in human vas deferens (Banks et al., 2006; Medina et al., 2010; Hedlund et al., 1985; Anton and McGrath, 1977) but also how the modulatory effects of specific K+ channels may be pivotal for purinergic contribution to overall contractile function of the tissue, which also involves action by co-released neurotransmitter, noradrenaline.
Where applicable, experiments conform with Society ethical requirements