Endogenous and exogenous agonist-induced changes in the coupling between [Ca2+]i and force in rat resistance arteries.
The relationship between isometric tension and free cytoplasmic calcium, [Ca2+]i, was investigated in rat isolated resistance arteries using fura-2. Depolarisation with 125 mM K+ induced a tonic contraction, while [Ca2+]i increased transiently but stabilised above resting [Ca2+]i. Furthermore, the tension/[Ca2+]i ratio was lower during activation with 125 mM K+ if the effect of endogenous noradrenaline (NA) was inhibited. Concentration/response curves with NA and K+ indicated that NA increased the sensitivity to [Ca2+]i. Calcium concentration/response curves in the presence of 10 microM NA or 125 mM K+ showed that NA could induce force at or below resting [Ca2+]i, while for any given bath calcium concentration, [Ca2+]i was similar in the presence of NA or K+. Addition of NA or vasopressin (AVP) to vessels depolarised with 125 mM K+ caused force development but no increase in [Ca2+]i, suggesting that agonists increase the efficacy of [Ca2+]i. However, during activation with AVP the efficacy of [Ca2+]i decreased time-dependently. The results suggest that in resistance arteries [Ca2+]i plays a crucial role in excitation-contraction coupling, but the tension/[Ca2+]i relationship can be modified by exogenous and endogenous agonists.
Jensen
P E
PE
Institute of Pharmacology, University of Aarhus, Denmark.
Mulvany
M J
MJ
Aalkjaer
C
C
D000818:Animals; D001127:Arginine Vasopressin / Q000494:pharmacology; D001696:Biomechanical Phenomena; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D009119:Muscle Contraction; D009131:Muscle, Smooth, Vascular / Q000378:metabolism*; D009638:Norepinephrine / Q000494:pharmacology; D011188:Potassium / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D012964:Sodium / Q000494:pharmacology; D014655:Vascular Resistance
D001127:Arginine Vasopressin; D012964:Sodium; D011188:Potassium; D002118:Calcium; D009638:Norepinephrine