Cooling enhances the contractile response to 5-HT in equine small lamellar arteries: effect of the endothelium.

Hector Zerpa1, Yoel Berhane1, Jonathan Elliott1, Simon R. Bailey2
1Royal Veterinary College, University of London, London, United Kingdom, 2Faculty of Veterinary Science, University of Melbourne, Victoria, Australia

The equine digital vascular bed has an important physiological role in thermoregulation. Cooling enhances 5-HT-receptor mediated vasoconstrictor responses in these cutaneous vessels (Zerpa et al., 2007). Whether this effect is also present in small cutaneous arteries from the equine hoof, is still unclear. The aim of this study was to establish the effect of cooling on the response of small lamellar arteries (SLA) (ID: 206±11μm) to 5-HT. SLA rings were harvested from adult mixed breed horses of either gender killed at an abattoir. Rings from the same horse (n=5) were studied simultaneously using wire myography. All vessels were stimulated initially with depolarising Krebs solution (DKS; KCl 118 mM). Subsequently, vessels were preconstricted with phenylephrine (10 μM) and then, the relaxant response to carbachol (CCh 1 μM) was evaluated The contractile effect of cumulatively increasing concentrations of 5-HT (0.1 nM-10 μM) was then assessed in endothelium intact (E+) or denuded vessels (E-), and in the absence or presence of L-NAME (300 μM) + Ibuprofen (10 μM) in E+ vessels, at temperatures of 30°C (the peripheral hoof temperature of horses maintained in a thermo-neutral environment) and 22°C. At the end of each experiment, the relaxant response to CCh (1 μM) was evaluated in 5-HT-preconstricted vessels. Data are presented as the increase in tension expressed as percentage of the DKS response, pD2 or percentage of relaxation. Statistical comparisons between groups were performed using paired or unpaired Student’s t tests.

The response to DKS was not significantly reduced by cooling or removal of the endothelium. The Emax for 5-HT at 22°C was greater than at 30°C in both E+ (217±19%DKS vs 170±19%DKS, at 22 and 30 °C, respectively; P= 0.07) and E- vessels (269±26%DKS vs 172±20%DKS; P=0.04). Mechanical removal of the endothelium increased the pD2 for 5-HT at both temperatures (-7.2 vs -7.6 at 30°C in E+ and E- vessels, respectively; P=0.02; and -7.2 vs -7.5 at 22°C; P=0.02). There was no effect of temperature on pD2 values. L-NAME+Ibuprofen increased the basal tension at 30°C (35±13%DKS) and 22°C (55±20%DKS). CCh (1μM) induced relaxation in E+ preparations only (86±5% at 30°C and 81±7% at 22°C; n=16). The control response to CCh was similar at both temperatures (91±4% at 30°C and 85±11% at 22°C; P=0.50). In contrast, the L-NAME+Ibuprofen-resistant response was significantly lower in cooled vessels (70±12% at 30°C and 13±4% at 22°C; P=0.006).

Cooling enhanced the contractile response to 5-HT in the SLA. This effect appeared to be more marked in E- vessels, indicating that cooling influenced the modulation of 5-HT contractile effects by the endothelium. This suggestion is supported by the observation that L-NAME+Ibuprofen increase basal tone in these vessels, an effect that tended to be greater at 22 than 30°C. L-NAME+Ibuprofen inhibited the relaxant response to CCh to a greater extent at 22°C than 30°C. Together, these data indicate that endothelial nitric oxide and/or prostacyclin production is up-regulated by cooling in these blood vessels.

Zerpa H et al., 2007. Eur J Pharmacol. 569:212-221.

HZ was supported by CDCH-UCV and the Programme Alβan, the EU scholarships for Latin America, scholarship No. E05D054410VE.