Impaired small-conductance Ca2+-activated K+ channel-dependent EDHF responses in type II diabetic ZDF rats

1 We have examined the relative contributions of small- and intermediate-conductance Ca2+-activated K+ channels (SKCa and IKCa) to the endothelium-derived hyperpolarizing factor (EDHF) pathway response in small mesenteric arteries of Zucker Diabetic Fatty (ZDF) rats, before and after the development of Type II diabetes, together with Lean controls. 2 Smooth muscle membrane potential was recorded using sharp microelectrodes in the presence of 10 mu M indomethacin plus 100 mu M N-omega-nitro-L-arginine. SKCa was selectively inhibited with 100 nM apamin, whereas IKCa was blocked with 10 mu M TRAM-39 (2-(2-chlorophenyl)-2,2-diphenylacetonitrile). 3 Resting membrane potentials were similar in arteries from 17- to 20-week-old control and diabetic rats (approximately -54 mV). Responses elicited by 1 and 10 mM acetylcholine (ACh) were significantly smaller in the diabetic group (e.g. hyperpolarizations to -69.5 +/- 0.8mV (ZDF; n = 12) and -73.2 +/- 0.6mV (Lean; n = 12; P < 0.05) evoked by 10 mu M ACh). 4 The IKCa-mediated components of the ACh responses were comparable between groups (hyperpolarizations to approximately -65mV on exposure to 10 mu M ACh). However, SKCa-mediated responses were significantly reduced in the diabetic group (hyperpolarizations to -63.1 +/- 1.0mV (ZDF; n = 6) and -71.5 +/- 1.2mV (Lean; n = 6; P < 0.05) on exposure to 10 mu M ACh. 5 Impaired ACh responses were not observed in arteries from 5- to 6-week- old (pre-diabetic) animals. SKCa subunit mRNA expression was increased in the diabetic group. 6 The EDHF pathway, especially the SKCa-mediated response, is impaired in Type II diabetic ZDF rats without a reduction in channel gene expression. These results may be particularly relevant to the microvascular complications of diabetes. The functional separation of SKCa and IKCa pathways is discussed.