Effects of substance P on [Ca2+]i and force in intact guinea pig ileal smooth muscle.
In intact sheets of the guinea pig ileal longitudinal muscle, loaded with fura-2, both substance P (SP)- and K(+)-induced contractile responses are preceded by cytoplasmic free Ca2+ ([Ca2+]i) variations. In response to SP (10(-10)-10(-7) M), [Ca2+]i and force increased concentration dependently. From 10(-9) M on, the response was biphasic: an initial Ca2+ spike and force transient were followed by a tonic component. The [Ca2+]i and force vs. log [SP] curves were sigmoidal for the initial phasic component, while a homologous receptor desensitization caused a reduced tonic component of the [Ca2+]i and contractile response at higher concentrations of SP. Both intracellular Ca2+ release and Ca2+ influx play a role in the effect of the peptide. In depolarizing solutions (140 mM K+) and in Ca2(+)-free medium (2 mM EGTA), SP induced a transient increase in [Ca2+]i and force. The Ca2+ stores used by SP and acetylcholine (ACh) overlap. [Ca2+]i and force fell to base-line levels when the extracellular Ca2+ was reduced from 1.2 to 0.2 mM during stimulation with SP. Verapamil reduced the tonic response. We also studied the relation between [Ca2+]i and force for the peak and steady-state values after stimulation with increasing concentrations of SP and K+. The phasic force response was linearly related to log [Ca2+]i. During the sustained response to K+, the Ca2+ sensitivity of the contractile/regulatory proteins was decreased, whereas no changes were observed during prolonged stimulation with the peptide.
Matthijs
G
G
Department of Medical Research, University of Leuven, Belgium.
Himpens
B
B
Peeters
T L
TL
Vantrappen
G
G
D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D003600:Cytosol / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D006168:Guinea Pigs; D007082:Ileum / Q000187:drug effects / Q000502:physiology*; D066298:In Vitro Techniques; D009119:Muscle Contraction / Q000187:drug effects*; D009130:Muscle, Smooth / Q000187:drug effects / Q000502:physiology*; D011188:Potassium / Q000494:pharmacology; D013373:Substance P / Q000494:pharmacology*; D013997:Time Factors; D014700:Verapamil / Q000494:pharmacology
D013373:Substance P; D014700:Verapamil; D011188:Potassium; D002118:Calcium