Hypotonic swelling-induced Ca(2+) release by an IP(3)-insensitive Ca(2+) store

Hypotonic swelling increases the intracellular Ca(2+) concentration ([Ca(2+)](i)) in vascular smooth muscle cells (VSMC). The source of this Ca(2+) is not clear. To study the source of increase in [Ca(2+)](i) in response to hypotonic swelling, we measured [Ca(2+)](i) in fura 2-loaded cultured VSMC (A7r5 cells). Hypotonic swelling produced a 40.7-nM increase in [Ca(2+)](i) that was not inhibited by EGTA but was inhibited by 1 microM thapsigargin. Prior depletion of inositol 1,4,5-trisphosphate (IP(3))-sensitive Ca(2+) stores with vasopressin did not inhibit the increase in [Ca(2+)](i) in response to hypotonic swelling. Exposure of (45)Ca(2+)-loaded intracellular stores to hypotonic swelling in permeabilized VSMC produced an increase in (45)Ca(2+) efflux, which was inhibited by 1 microM thapsigargin but not by 50 microg/ml heparin, 50 microM ruthenium red, or 25 microM thio-NADP. Thus hypotonic swelling of VSMC causes a release of Ca(2+) from the intracellular stores from a novel site distinct from the IP(3)-, ryanodine-, and nicotinic acid adenine dinucleotide phosphate-sensitive stores.