Contraction of rat isolated tail arteries induced by a hypo-osmotic solution is dependent on protein tyrosine kinases and activation of Cav1.2 calcium channels S.Wijetunge & A. D. Hughes, Clinical Pharmacolgy, NHLI Division, Faculty of Medicine, Imperial College London, W2 1NY UK.	Print Abstract Search PubMed for: Wijetunge S Hughes AD

Blood vessels respond to elevated transmural pressure or stretching by contracting. At the cellular level, this contraction is attributable to an increase in intracellular calcium concentration, possibly via L-type voltage-operated calcium channels (Ca_V1.2) (McCarron et al., 1997). The biochemical mechanism leading to stretch-induced activation of Ca_V1.2 is not clear, although protein tyrosine kinases (PTK) may play a role. The aim of this study was to investigate the role of PTK and Ca_V1.2 in arterial contraction induced by a hypo-osmotic solution in vitro.

Isometric tension was measured in rat isolated tail arteries mounted in a myograph containing a modified Kreb's physiological saline solution (PSS) containing 50mM mannitol. PSS was gassed with 95% O₂ and 5% CO₂ and maintained at 37ºC. Basal tone (10-20% of maximum contraction) was induced by either increasing [K⁺] in PSS to 20-40mM or addition of a thromboxane mimetic, U46619 (200-300nM). Hyposmotic swelling was induced by reducing the concentration of mannitol in PSS. All responses are expressed as % response to PSS containing isomolar substitution of 118mM K⁺ for Na⁺ (K_max). Statistical comparisons were made using a paired Student's t-test. P<0.05 was considered significant.

A reduction in [mannitol] caused a concentration-dependent contraction of rat isolated tail arteries (Table).

Table: contraction of rat isolated tail artery in different solutions (% K_max). Data are mean±SEM of (n) observations.

Pre-incubation of arteries with verapamil (20µM), a blocker of Ca_V1.2, inhibited the contractile response to 0mM mannitol by 86 ± 3%; p < 0.001, but had no effect on basal tone (control= 23 ± 5% and verapamil =24 ± 5%; n=11, p=0.58).

Pre-incubation with genistein (10mM), an inhibitor of PTK reduced basal tone from 7 ± 1% to 1 ± 1% and inhibited the contractile response to a hyposmotic stimulus by 92 ± 4% (n=15, p < 0.001). Daidzein (10µM), an inactive analogue of genistein did not significantly affect responses to the hyposmotic stimulus (% inhibition = 29 ±16%; n=10, p=0.11).

The results indicate that contraction induced by a hyposmotic solution is largely dependent on calcium influx through Ca_V1.2 and that a PTK may be involved in this response.

McCarron et al., (1997). J.Physiol, 498, 371-379.

Acknowledgement: This study was supported by the British Heart Foundation.