Temporal changes in the calcium-force relation during histamine-induced contractions of strips of the coronary artery of the pig

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Abstract
1 We examined temporal changes in the relationship between cytosolic calcium concentrations ([Ca2+]i) and developed tension during histamine-induced contractions of strips of the coronary artery of the pig, by making use of simultaneous measurements of fura-2 fluorescence and force. 2 The relationship between [Ca2+]i and developed tension observed with cumulative applications of extracellular Ca2+ ([Ca2+]o), ranging from 0 mm to 10 mm, during 118 mm K+-depolarization was similar to that observed in chemically skinned strips of the porcine coronary artery, as noted by other investigators. [Ca2+]i at 0 mm [Ca2+]o, at 50% of maximum, and at maximum tension development were 76 nm, 424 nm, and 3050 nm, respectively. 3 Cumulative applications of histamine induced dose-dependent increases in [Ca2+]i and tension and the extent of tension for a given change in [Ca2+]i increased, i.e. greater effectiveness of [Ca2+]i-tension relationship, than seen with K+-depolarization. 4 When histamine 10−5 m was applied, [Ca2+]i abruptly rose and reached the first peak within several seconds. After a slight dip at 30 s, [Ca2+]i reached a second peak at 3 min, and then gradually declined. On the other hand, tension developed rapidly reached a maximum at 4 min, then gradually declined. The relation between [Ca2+]i and tension in the early, rising phase of contraction was similar to that obtained during depolarization. At the time of maximum tension development, the relation was greater than that observed during depolarization, which persisted in the phase of declining tension. 5 To examine the role of protein kinase C in the increased effectiveness of [Ca2+]i-tension relation of histamine-induced contractions, we used 1-(5-isoquinolinesulphonyl)-2-methylpiperazine (H-7), as an inhibitor. Exposure of strips to H-7, 10−5 m, reduced the effectiveness of [Ca2+]i-tension relation toward the level observed during depolarization, while it had no effect on the [Ca2+]i-tension relation in the early phase of contraction either in the presence or absence of extracellular Ca2+. 6 In intact smooth muscle of the coronary artery of the pig, the relationship between [Ca2+]i and developed tension varies during contraction. Histamine, a receptor-mediated stimulus, induced a greater tension for a given change in [Ca2+]i during the steady state of contraction than did depolarization. The greater effectiveness of contraction, particularly in the later phase, seems to be mediated by activation of an H-7-sensitive mechanisms.

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