Relationship between force and Ca2+ in anococcygeal and vas deferens smooth muscle cells of the mouse.
We compared the changes of the cytoplasmic Ca2+ concentration ([Ca2+]i), as measured with the fluorescent Ca2+ indicator fura-2, and the force development in intact smooth muscle of the tonic anococcygeus (AC) and the phasic vas deferens (VD) of the mouse, during activation by K+ depolarization and by agonists. Resting [Ca2+]i was observed to be 33% lower in AC (80 nM) than in VD (115 nM), while the Ca2+ threshold for contraction was found to be about 120 nM in AC and 160 nM in VD. For a similar [Ca2+]i increase, the agonist stimulation induced a higher force development than the K+ depolarization in both muscle types. During prolonged depolarization, the force/calcium ratio increased in AC but strongly declined in VD. This decline of the force/calcium ratio in VD during depolarization was partially reversed by lowering [Ca2+]o. Our results indicate that the Ca2+ threshold for force development was about 150% of the resting [Ca2+]i in both cell types. The resting [Ca2+]i was lower in the tonic AC than in the phasic VD. Agonist-induced sensitization to Ca2+ occurred in both muscle types. The tonic and phasic smooth muscles essentially differed in the respective modulation of their Ca2+ sensitivity during contraction. The desensitization to Ca2+ was specific for phasic muscle, in which it occurred as an early, time- and Ca(2+)-dependent process that was partially reversible.
Boland
B
B
Department of Physiology, Université Catholique de Louvain, Bruxelles, Belgium.
Himpens
B
B
Gillis
J M
JM
Casteels
R
R
D000818:Animals; D002118:Calcium / Q000502:physiology*; D004558:Electric Stimulation; D066298:In Vitro Techniques; D007700:Kinetics; D008297:Male; D051379:Mice; D009119:Muscle Contraction / Q000502:physiology*; D009130:Muscle, Smooth / Q000502:physiology*; D011188:Potassium / Q000502:physiology; D014649:Vas Deferens / Q000502:physiology*
D011188:Potassium; D002118:Calcium