Desensitization to cytoplasmic Ca2+ and Ca2+ sensitivities of guinea-pig ileum and rabbit pulmonary artery smooth muscle.
1. The free cytoplasmic Ca2+ concentration [( Ca2+]i) was measured in the tonic rabbit pulmonary artery and the phasic ileum smooth muscle. 2. Force development and [Ca2+]i were determined during either cumulative or non-cumulative additions of [Ca2+]o to smooth muscles depolarized with 140 mM-K+ solutions. 3. The level to which [Ca2+]i declined in Ca2+-free, 140 mM-K+ solutions was significantly lower in the ileum (40 +/- 4 nM) than in pulmonary artery (77 +/- 5 nM) smooth muscle. 4. The level of [Ca2+]i reached during non-cumulative superfusion with 10 microM and 1 mM [Ca2+]o was higher in the pulmonary artery than in the ileum. 5. The force level reached for a given [Ca2+]i was also higher in the pulmonary artery than in the ileum. 6. During maintained depolarization there was a marked decrease in the sensitivity of ileum smooth muscle tension to [Ca2+]i. 7. We conclude that significant differences exist in the Ca2+ sensitivity of the regulatory/contractile apparatus among different smooth muscles; the lower sensitivity of depolarized ileum than pulmonary artery to [Ca2+]o is due to both differences in Ca2+ metabolism and in the Ca2+ sensitivity of the regulatory contractile system. We suggest that these two mechanisms also contribute to the decline in force during a phasic K+ contracture, and that desensitization to [Ca2+]i contributes to the decline of the K+ contracture in the ileum.
Himpens
B
B
Pennsylvania Muscle Institute, University of Pennsylvania School of Medicine, Philadelphia 19104-6083.
Matthijs
G
G
Somlyo
A P
AP
D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D003593:Cytoplasm / Q000378:metabolism*; D005260:Female; D007082:Ileum / Q000378:metabolism; D066298:In Vitro Techniques; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects*; D009130:Muscle, Smooth / Q000378:metabolism*; D009131:Muscle, Smooth, Vascular / Q000378:metabolism*; D011651:Pulmonary Artery / Q000378:metabolism; D011817:Rabbits
D002118:Calcium