Ca2+ channel currents and contraction in CaVβ3-deficient ileum smooth muscle from mouse

Voltage activated L-type Ca2+ channels are the principal Ca2+ channels in intestinal smooth muscle cells. They comprise the ion conducting CaV1 pore and the ancillary subunits α2δ and β. Of the four CaVβ subunits CaVβ3 is assumed to be the relevant CaVβ protein in smooth muscle. In protein lysates isolated from mouse ileum longitudinal smooth muscle we could identify the CaV1.2, CaVα2, CaVβ2 and CaVβ3 proteins, but not the CaVβ1 and CaVβ4 proteins. Protein levels of CaV1.2, CaVα2 and CaVβ2 are not altered in ileum smooth muscle obtained from CaVβ3-deficient mice indicating that there is no compensatory increase of the expression of these channel proteins. Neither the CaVβ2 nor the other CaVβ proteins appear to substitute for the lacking CaVβ3. L-type Ca2+ channel properties including current density, inactivation kinetics as well as Cd2+- and dihydropyridine sensitivity were identical in cells of both genotypes suggesting that they do not require the presence of a CaVβ3 protein. However, a key hallmark of the CaVβ modulation of Ca2+ current, the hyperpolarisation of channel activation is slightly but significantly reduced by 4 mV. In addition to L-type Ca2+ currents T-type Ca2+ currents could be recorded in the murine ileum smooth muscle cells, but T-type currents were not affected by the lack of CaVβ3. Both proteins, CaVβ2 and CaVβ3 are localized near the plasma membrane and the localization of CaVβ2 is not altered in CaVβ3 deficient cells. Spontaneous contractions and potassium and carbachol induced contractions are not significantly different between ileum longitudinal smooth muscle strips from mice of both genotypes. In summary the data show that in ileum smooth muscle cells, CaVβ3 has only subtle effects on L-type Ca2+ currents, appears not to be required for spontaneous and potassium induced contraction but might have a function beyond being a Ca2+ channel subunit.