Cytoplasmic Ca2+ mobilization and Ca(2+)-dependent membrane currents in dispersed bovine ciliary muscle cells.
OBJECTIVE
The dependence of plasmalemma Ca2+ influx and Ca2+ release from intracellular stores on Ca2+ activated K+ channels of bovine ciliary muscle (CM) cells were examined.


METHODS
The nystatin-perforated patch clamp technique for the measurement of membrane currents and a microscope based fura-2 fluorescence imaging of [Ca2+]i were applied to CM cells freshly dissociated with collagenase and identified with smooth muscle-specific alpha-isoactin.


RESULTS
At holding voltages (VH) of > -60 mV, CM cells showed spontaneous transient outward currents (STOCs) and caffeine (> 10(-4) M) induced large transient outward currents (ICAF). Both STOCs and ICAF were abolished by tetraethylammonium chloride (10(-3) M) and charybdotoxin (10(-7) M), but not by apamin (10(-6) M), suggesting that both currents are mediated by Ca(2+)-activated K+ channels similar to those with medium (MK) or large (BK-type) conductance. Both STOCs and ICAF were gradually abolished in the nominally Ca(2+)-free and Co(2+)-containing solutions but were resistant to L-type Ca2+ channel blockers, including nicardipine, verapamil and diltiazem and a N-type channel blocker, omega-contoxin. The [Ca2+]i-elevation during high K+ (100 mM)-depolarization was prevented by Ca(2+)-free and Co(2+)-containing solutions but not by nicardipine.


CONCLUSIONS
These results suggest that CM cells possess MK or BK type-like Ca(2+)-activated K+ channels and that L-type Ca2+ channels play minor roles for the maintenance of Ca(2+)-dependent responses in contrast to other types of smooth muscle cells.
Fujii
T
T
Department of Ophthalmology, Kumamoto University School of Medicine, Japan.
Tokutomi
N
N
Hirata
A
A
Negi
A
A
Nishi
K
K
D000199:Actins / Q000378:metabolism; D000818:Animals; D001692:Biological Transport; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism* / Q000502:physiology*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002217:Carbachol / Q000494:pharmacology; D002417:Cattle; D002469:Cell Separation; D002924:Ciliary Body / Q000166:cytology / Q000378:metabolism / Q000502:physiology*; D003593:Cytoplasm / Q000378:metabolism*; D004553:Electric Conductivity; D005455:Fluorescent Antibody Technique; D009130:Muscle, Smooth / Q000166:cytology / Q000378:metabolism / Q000502:physiology*; D009994:Osmolar Concentration; D018408:Patch-Clamp Techniques
D000199:Actins; D002121:Calcium Channel Blockers; D002110:Caffeine; D002217:Carbachol; D002118:Calcium