Molecular constituents of maxi K(Ca) channels in human coronary smooth muscle: Predominant α + β subunit complexes

Abstract

1. Human large-conductance voltage- and calcium-sensitive K+ (maxi K(Ca)) channels are composed of at least two subunits: the pore-forming subunit, α, and a modulatory subunit, β. Expression of the β subunit induces dramatic changes in α subunit function. It increases the apparent Ca2+ sensitivity and it allows dehydrosoyasaponin I (DHS-I) to upregulate the channel. 2. The functional coupling of maxi K(Ca) channel α and β subunits in freshly dissociated human coronary smooth muscle cells was assessed. To distinguish maxi K(Ca) currents modulated by the β subunit, we examined (a) their apparent Ca2+ sensitivity, as judged from the voltage necessary to half-activate the channel (V( 1/4 )), and (b) their activation by DHS-I. 3. In patches with unitary currents, the majority of channels were half-activated near -85 mV at 18 μM Ca2+, a value similar to that obtained when the human K(Ca) channel α (HSLO) and β (HK(V,Ca)β) subunits are co-expressed. A small number of channels half-activated around 0 mV, suggesting the activity of the α subunit alone. 4. The properties of macroscopic currents were consistent with the view that most pore-forming α subunits were coupled to β subunits, since the majority of currents had values for V( 1/4 ) near to -90 mV, and currents were potentiated by DHS-I. 5. We conclude that in human coronary artery smooth muscle cells, most maxi K(Ca) channels are composed of α and β subunits. The higher Ca2+ sensitivity of maxi K(Ca) channels, resulting from their coupling to β subunits, suggests an important role of this channel in regulating coronary tone. Their massive activation by micromolar Ca2+ concentrations may lead to a large hyperpolarization causing profound changes in coronary blood flow and cardiac function.