Functional and molecular evidence of MaxiK channel β1 subunit decrease with coronary artery ageing in the rat

摘要

Large-conductance, voltage- and Ca²⁺-activated K⁺ channels (MaxiK, BK) are key regulators of vascular tone. Vascular MaxiK are formed by the pore-forming α subunit and the modulatory β1 subunit, which imprints unique kinetics, Ca²⁺/voltage sensitivities and pharmacology to the channel. As age progresses, α subunit functional expression and protein levels diminish in coronary myocytes. However, whether ageing modifies β1 subunit expression or the mechanism of α subunit reduction is unknown. Thus, we examined functional and pharmacological characteristics of MaxiK, as well as α and β1 transcript levels in coronary myocytes from young and old F344 rats. The mechanism of age-dependent α subunit protein reduction involves its transcript downregulation. A corresponding loss of β1 transcripts was also detected in old myocytes, suggesting a proportional age-dependent decrease of β1 to α subunit protein. Indeed, MaxiK channel properties, defined by coassembly of β1 and α subunits, were equivalent in young versus old, for example in terms of (i) activation kinetics, (ii) sensitivity to Ca²⁺ levels > 1 μm (iii) dehydrosoyasaponin-I-induced activation, and (iv) iberiotoxin blockade. Consistent with less MaxiK expression/function in older myocytes, the ability of iberiotoxin to contract coronary rings was reduced ∼50% with ageing confirming our previous findings. 5-Hydroxytryptamine (5-HT) contractile efficacy was reduced by iberiotoxin pretreatment in young > old coronary arteries (explained by larger iberiotoxin-induced contraction and decreased dynamic range for 5-HT contraction in young versus old) with no apparent differences in nitroglycerine-induced relaxation. We propose that the age-related MaxiK reduction involves a parallel decrease of α and β1 functional expression via a transcript downregulatory mechanism; a major impact on basal and possibly stimulated coronary contraction may contribute to altered coronary flow regulation and coronary morbidity in the elderly.