Mechanism of carbachol-evoked contractions of guinea-pig ileal smooth muscle close to freezing point.
1. The effect of lowering the temperature to near freezing-point upon the contractions and [3H]-inositol phosphate responses to carbachol were investigated in longitudinal smooth muscle from the guinea-pig ileum. 2. The peak amplitude of the contraction to a single application of 100 microM carbachol was the same at 37 degrees C and temperatures near freezing-point. However, the sensitivity to carbachol was reduced upon lowering the temperature and the time to peak contraction was increased from 5-10 s to 2-10 min. Even when the temperature was maintained near freezing-point, washing off carbachol produced a relaxation and eventual return of tension to basal levels. 3. Incubating the tissue in 140 mM K+, calcium-free solution or in calcium channel antagonists significantly reduced the carbachol-induced contraction to 10-30% of the control at 37 degrees C and also at 3 degrees C. Thus the majority of the activator calcium required for contraction entered the tissue via voltage-dependent calcium channels (VDCs) at both 37 degrees C and 3 degrees C. 4. The contractions produced by high potassium solutions were less at temperatures close to freezing-point than those at 37 degrees C suggesting that voltage-dependent calcium entry was inhibited as the temperature was lowered. 5. A small part of the contractile response to 100 microM carbachol was resistant to the removal of extracellular calcium at both 37 degrees C and 3 degrees C and this component was increased under depolarizing conditions. This suggests that the release of stored calcium contributes to a minor degree to contraction at both 37 degrees C and 3 degrees C.6. Although 100 microM carbachol produced a statistically significant rise in several [3H]-inositol phosphate isomers at both 37 degrees C and 3 degrees C, the production of [3H]-inositol phosphates was less at 3 degrees C than at 37 degrees C and the increase in their production caused by carbachol was much slower.7. These results suggest that the carbachol-induced contraction at 3 degrees C utilizes both calcium entry through VDCs and calcium release from intracellular stores, as at 37 degrees C. The components of the responses dependent upon intracellular calcium release at 37 degrees C and at temperatures near freezing-point were similar. However, the production of [3H]-inositol phosphates, including the calcium-mobilizing second messenger inositol (1,4,5) trisphosphate (Ins(1,4,5)P3), is reduced at such low temperatures.
Blackwood
A M
AM
Department of Pharmacology and Clinical Pharmacology, St George's Hospital Medical School, London.
Bolton
T B
TB
D000818:Animals; D001285:Atropine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism / Q000502:physiology; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002217:Carbachol / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D003080:Cold Temperature; D005260:Female; D006168:Guinea Pigs; D007082:Ileum / Q000187:drug effects; D066298:In Vitro Techniques; D007295:Inositol Phosphates / Q000378:metabolism; D007537:Isometric Contraction / Q000187:drug effects; D009130:Muscle, Smooth / Q000187:drug effects*; D009467:Neuromuscular Depolarizing Agents / Q000494:pharmacology; D011188:Potassium / Q000494:pharmacology
D002121:Calcium Channel Blockers; D007295:Inositol Phosphates; D009467:Neuromuscular Depolarizing Agents; D001285:Atropine; D002217:Carbachol; D011188:Potassium; D002118:Calcium