Calcium release events in excitation-contraction coupling in smooth muscle.
Although smooth muscle cells are not organized in sarcomeres, as are striated muscles, nevertheless Ca2+ for contraction is released from the sarcoplasmic reticulum (SR) at certain preferred sites. These sites commonly discharge packets of Ca2+ spontaneously and have been called frequent discharge sites (FDSs). Each spontaneous release of a Ca2+ packet usually leads to a burst of openings of Ca2+-activated K+ channels in the cell membrane which produces a spontaneous transient outward current (STOC) in smooth muscle cells under voltage clamp. When fluorescent Ca2+ indicators such as Fluo-3 became available, the spontaneous transient increases in [Ca2+]i produced by Ca2+ packets released from the SR were also detected in cardiac muscle as flashes of fluorescence or 'sparks'. Sparks in smooth muscle consist of smaller Ca2+ packets that can give rise to 'microsparks'. In some smooth muscles which have Ca2+-activated Cl- channels, STICs (spontaneous transient inward currents) are also found to be associated with sparks. FDSs have been found to be important initiating sites for a Ca2+ wave in response to an action potential or in response to receptor activation and possibly other stimuli, such as stretch. In both cases Ca2+-induced Ca2+ release seems to be crucially involved.
Bolton
T B
TB
Department of Pharmacology & Clinical Pharmacology, St George's Hospital Medical School, London, UK.
Gordienko
D V
DV
Pucovský
V
V
Parsons
S
S
Povstyan
O
O
D000818:Animals; D002118:Calcium / Q000502:physiology*; D007082:Ileum / Q000502:physiology; D008322:Mammals; D008564:Membrane Potentials / Q000502:physiology; D009119:Muscle Contraction / Q000502:physiology*; D009126:Muscle Relaxation / Q000502:physiology; D009130:Muscle, Smooth / Q000502:physiology*; D009131:Muscle, Smooth, Vascular / Q000502:physiology; D012518:Sarcomeres / Q000502:physiology
D002118:Calcium