Inositol 1,4,5-trisphosphate receptors modulate Ca2+ sparks and Ca2+ store content in vas deferens myocytes.
Spontaneous Ca2+ sparks were observed in fluo 4-loaded myocytes from guinea pig vas deferens with line-scan confocal imaging. They were abolished by ryanodine (100 microM), but the inositol 1,4,5-trisphosphate (IP3) receptor (IP3R) blockers 2-aminoethoxydiphenyl borate (2-APB; 100 microM) and intracellular heparin (5 mg/ml) increased spark frequency, rise time, duration, and spread. Very prolonged Ca2+ release events were also observed in approximately 20% of cells treated with IP3R blockers but not under control conditions. 2-APB and heparin abolished norepinephrine (10 microM; 0 Ca2+)-evoked Ca2+ transients but increased caffeine (10 mM; 0 Ca2+) transients in fura 2-loaded myocytes. Transients evoked by ionomycin (25 microM; 0 Ca2+) were also enhanced by 2-APB. Ca2+ sparks and transients evoked by norepinephrine and caffeine were abolished by thimerosal (100 microM), which sensitizes the IP3R to IP3. In cells voltage clamped at -40 mV, spontaneous transient outward currents (STOCs) were increased in frequency, amplitude, and duration in the presence of 2-APB. These data are consistent with a model in which the Ca2+ store content in smooth muscle is limited by tonic release of Ca2+ via an IP3-dependent pathway. Blockade of IP3Rs elevates sarcoplasmic reticulum store content, promoting Ca2+ sparks and STOC activity.
White
Carl
C
Smooth Muscle Research Group, School of Medicine, Queen's University of Belfast, Belfast BT9 7BL, UK.
McGeown
J Graham
JG
D000818:Animals; D000891:Anti-Infective Agents, Local / Q000494:pharmacology; D000925:Anticoagulants / Q000494:pharmacology; D001896:Boron Compounds / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000378:metabolism*; D005110:Extracellular Space / Q000378:metabolism; D025461:Feedback, Physiological / Q000502:physiology; D006168:Guinea Pigs; D006493:Heparin / Q000494:pharmacology; D053496:Inositol 1,4,5-Trisphosphate Receptors; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D032389:Myocytes, Smooth Muscle / Q000378:metabolism*; D018160:Receptors, Cytoplasmic and Nuclear / Q000037:antagonists & inhibitors / Q000378:metabolism*; D012433:Ryanodine / Q000494:pharmacology; D012519:Sarcoplasmic Reticulum / Q000378:metabolism; D013849:Thimerosal / Q000494:pharmacology; D014649:Vas Deferens / Q000166:cytology* / Q000378:metabolism
D000891:Anti-Infective Agents, Local; D000925:Anticoagulants; D001896:Boron Compounds; D015220:Calcium Channels; D053496:Inositol 1,4,5-Trisphosphate Receptors; D018160:Receptors, Cytoplasmic and Nuclear; D012433:Ryanodine; D013849:Thimerosal; D006493:Heparin; C109986:2-aminoethoxydiphenyl borate; D002118:Calcium