Molecular constituents of maxi K-Ca channels in human coronary smooth muscle: predominant alpha+beta subunit complexes

1. Human large-conductance voltage-and calcium-sensitive K+ (maxi K-Ca) channels are composed of at least two subunits: the pore-forming subunit, alpha, and a modulatory subunit, beta. Expression of the beta subunit induces dramatic changes in a 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 alpha and beta subunits in freshly dissociated human coronary smooth muscle cells was assessed. To distinguish maxi K-Ca currents modulated by the beta subunit, we examined (a) their apparent Ca2+ sensitivity, as judged from the voltage necessary to half-activate the channel (V-1/2), 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 mu M Ca2+, a value similar to that obtained when the human K-Ca channel alpha (HSLO) and beta (HK(V,Ca)beta) subunits are co-expressed. A small number of channels half-activated around 0 mV, suggesting the activity of the alpha subunit alone. 4. The properties of macroscopic currents were consistent with the view that most pore-forming alpha subunits were coupled to beta subunits, since the majority of currents had values for V-1/2 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 alpha and beta subunits. The higher Ca2+ sensitivity of maxi K-Ca channels, resulting from their coupling to beta 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.