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

Abstract

1 We have examined the relative contributions of small‐ and intermediate‐conductance Ca²⁺‐activated K⁺ channels (SK_Ca and IK_Ca) 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 μM indomethacin plus 100 μM N^ω‐nitro‐L‐arginine. SK_Ca was selectively inhibited with 100 nM apamin, whereas IK_Ca was blocked with 10 μM 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 μM acetylcholine (ACh) were significantly smaller in the diabetic group (e.g. hyperpolarizations to −69.5±0.8 mV (ZDF; n=12) and −73.2±0.6 mV (Lean; n=12; PμM ACh). 4 The IK_Ca‐mediated components of the ACh responses were comparable between groups (hyperpolarizations to approximately −65 mV on exposure to 10 μM ACh). However, SK_Ca‐mediated responses were significantly reduced in the diabetic group (hyperpolarizations to −63.1±1.0 mV (ZDF; n=6) and −71.5±1.2 mV (Lean; n=6; PμM ACh. 5 Impaired ACh responses were not observed in arteries from 5‐ to 6‐week‐old (pre‐diabetic) animals. SK_Ca subunit mRNA expression was increased in the diabetic group. 6 The EDHF pathway, especially the SK_Ca‐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 SK_Ca and IK_Ca pathways is discussed. British Journal of Pharmacology (2006) 148, 434–441. doi:10.1038/sj.bjp.0706748