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

Area of Science:

• Cell Biology
• Physiology
• Biochemistry

Background:

• Hypotonic swelling is known to elevate intracellular calcium (Ca2+) in vascular smooth muscle cells (VSMC).
• The precise origin of this Ca2+ influx remains incompletely understood.
• Understanding Ca2+ signaling is crucial for vascular function and disease.

Purpose of the Study:

• To investigate the source of intracellular Ca2+ ([Ca2+]i) increase induced by hypotonic swelling in cultured VSMC.
• To differentiate between extracellular and intracellular Ca2+ sources.
• To identify the specific intracellular Ca2+ store involved.

Main Methods:

• Measurement of intracellular Ca2+ ([Ca2+]i) using fura 2 in cultured VSMC (A7r5 cells).
• Experiments involving EGTA, thapsigargin, and vasopressin to probe Ca2+ sources and stores.
• Assessment of 45Ca2+ efflux from permeabilized VSMC under hypotonic conditions.

Main Results:

• Hypotonic swelling caused a significant increase in [Ca2+]i (40.7 nM) that was independent of extracellular Ca2+ (EGTA-insensitive).
• The Ca2+ increase was sensitive to thapsigargin, indicating involvement of intracellular stores.
• Depletion of inositol 1,4,5-trisphosphate (IP3)-sensitive stores did not affect the swelling-induced [Ca2+]i rise.
• 45Ca2+ efflux from intracellular stores under hypotonic conditions was inhibited by thapsigargin but not by heparin, ruthenium red, or thio-NADP.

Conclusions:

• Hypotonic swelling triggers Ca2+ release from intracellular stores in VSMC.
• This Ca2+ release originates from a novel intracellular store.
• The identified store is distinct from IP3-, ryanodine-, and nicotinic acid adenine dinucleotide phosphate (NAADP)-sensitive Ca2+ channels.