Type 1 IP(3) receptors activate BK(Ca) channels via local molecular coupling in arterial smooth muscle cells

Plasma membrane large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels and sarcoplasmic reticulum inositol 1,4,5-trisphosphate (IP(3)) receptors (IP(3)Rs) are expressed in a wide variety of cell types, including arterial smooth muscle cells. Here, we studied BK(Ca) channel regulation by IP(3) and IP(3)Rs in rat and mouse cerebral artery smooth muscle cells. IP(3) activated BK(Ca) channels both in intact cells and in excised inside-out membrane patches. IP(3) caused concentration-dependent BK(Ca) channel activation with an apparent dissociation constant (K(d)) of ∼4 µM at physiological voltage (−40 mV) and intracellular Ca(2+) concentration ([Ca(2+)](i); 10 µM). IP(3) also caused a leftward-shift in BK(Ca) channel apparent Ca(2+) sensitivity and reduced the K(d) for free [Ca(2+)](i) from ∼20 to 12 µM, but did not alter the slope or maximal P(o). BAPTA, a fast Ca(2+) buffer, or an elevation in extracellular Ca(2+) concentration did not alter IP(3)-induced BK(Ca) channel activation. Heparin, an IP(3)R inhibitor, and a monoclonal type 1 IP(3)R (IP(3)R1) antibody blocked IP(3)-induced BK(Ca) channel activation. Adenophostin A, an IP(3)R agonist, also activated BK(Ca) channels. IP(3) activated BK(Ca) channels in inside-out patches from wild-type (IP(3)R1(+/+)) mouse arterial smooth muscle cells, but had no effect on BK(Ca) channels of IP(3)R1-deficient (IP(3)R1(−/−)) mice. Immunofluorescence resonance energy transfer microscopy indicated that IP(3)R1 is located in close spatial proximity to BK(Ca) α subunits. The IP(3)R1 monoclonal antibody coimmunoprecipitated IP(3)R1 and BK(Ca) channel α and β1 subunits from cerebral arteries. In summary, data indicate that IP(3)R1 activation elevates BK(Ca) channel apparent Ca(2+) sensitivity through local molecular coupling in arterial smooth muscle cells.