Reduction of SKCa-mediated endothelium-dependent hyperpolarisations in mesenteric arteries from spontaneously-hypertensive rats

Ellen L. Porter, Arthur H. Weston, Owen Scudamore, Gillian Edwards

The University of Manchester, Manchester, UK

Stimulation of the vascular endothelium can trigger the opening of small- and intermediate-conductance calcium-sensitive potassium channels (SKCa and IKCa, respectively). By several mechanisms, this leads to hyperpolarisation and relaxation of the underlying myocytes and thus results in a reduction of blood vessel tone and lowering of arterial blood pressure (Busse et al., 2002). Since angiotensin II-induced hypertension may be associated with the loss of SKCa channel activity (Hilgers & Webb, 2007), the aim of the present study was to determine whether SKCa activity is reduced in the mesenteric arteries of spontaneously-hypertensive rats (SHRs).

Sharp microelectrode recordings (methods previously described in Edwards et al., 1999) were used to assess the role of SKCa in rat mesenteric artery segments (250-350μm external diameter) from male SHRs (10-14 weeks old) and age-matched Wistar-Kyoto (WKY) controls (all 200-325g). Results are expressed as mean ± s.e.mean and were analysed using ANOVA or Student’s t-tests as appropriate; P<0.05 was considered significant. Using the tail-cuff method, SHRs were shown to be hypertensive prior to being killed (systolic: SHR, 240 ± 9mmHg, n=9; WKY, 180 ± 4mmHg, n=10; diastolic: SHR, 160 ± 9mmHg, n=9; WKY, 110 ± 9mmHg, n=9) but with heart rates and body weights comparable to those of WKY controls. Impaled SHR myocytes showed a decreased resting membrane potential (-48.8 ± 0.5mV, n=4) compared with WKY (-52.9 ± 0.8mV, n=4) and significantly smaller hyperpolarisations in the presence of 10μM acetylcholine (WKY 19.0 ± 1.6mV, n=4, SHR 11.9 ± 1.5mV, both n=4). However, in the presence of 100nM apamin (to abolish the SKCa component of the acetylcholine response) acetylcholine-induced hyperpolarisations were similar in both WKY and SHR arteries (11.0 ± 0.5mV and 11.3 ± 1.0mV, respectively, both n=4), suggesting no change in the IKCa-mediated component. The hyperpolarisation induced by the SKCa opener CYPPA (30μM) was also smaller in arteries from hypertensive animals (WKY 19.4 ± 0.4mV; SHR 8.8 ± 0.9mV; both n=4). SDS-PAGE/Western blot analysis of SKCa alpha subunit expression (normalised relative to b-actin) showed no protein reduction. Thus it does not appear that SKCa channel expression is a contributory factor in hypertension although loss of SKCa channel activity resulting, for example, from impaired trafficking of the channel to the endothelial cell surface may be a factor. However, SKCa is functionally coupled to the inwardly rectifying potassium channel Kir2.1 (Harno et al., 2008), and thus further investigation into the possible role of Kir2.1 channel activity in the signalling pathway is required.

This study was funded by the British Heart Foundation

Edwards et al. (1999) Br J Pharmacol 128: 1788-94

Busse et al. (2002) Trends Pharmacol Sci 23: 374-80

Harno et al., 2008Cell Calcium 44: 210-9

Hilgers & Webb (2007). Am J Physiol 292: 2275-84