Figure 3. Role of Ca²⁺ in 5-HT-induced enhanced vasoconstriction in cultured coronary arteries.

A, Representative recording of 5-HT-evoked concentration-dependent contraction in cultured rat coronary arteries without endothelia in the presence or absence of nifedipine (left panel). 5-HT-induced contractions were significantly suppressed by nifedipine, but inhibition was incompletely. B, High K⁺-induced contractions were first reversed with nifedipine, but subsequent addition of 5-HT could still induce contractions. 5-HT-induced contraction was blocked by the non-selective CRAC channel blocker 2-APB (50 mM) (left panel). 2-APB significantly completely inhibited the 5-HT-induced contraction in the presence of nifedipine. C, Representative recording of 5-HT-evoked concentration-dependent contraction in fresh and cultured rat coronary arteries without endothelia in the presence of nifedipine (left panel). 5-HT induced greater contractions in the presence of nifedipine in cultured coronary arteries (full circles) than in fresh coronary arteries (open circles). D, Vasoconstrictions in response to Ca²⁺ influx after depletion of intracellular Ca²⁺ stores were evaluated. Rat coronary arterial rings were stimulated with 2 µM 5-HT for 15 minutes. Then rings were washed in Ca²⁺-free EGTA buffer to deplete intracellular Ca²⁺ stores for 20 minutes. During the depletion period, arteries were treated with 1 µM thapsigargin. After Ca²⁺ depletion, intracellular Ca²⁺ stores were loaded by 1.6 mM of Ca²⁺ buffer for 15 minutes. Contractile responses during the loading period were determined (left panel). Contractions induced by thapsigargin were augmented in cultured coronary arteries (black box) compared with fresh coronary arteries (white box) during the Ca²⁺-loading period (right panel). The graph shows the mean ± SEM of 6–10 experiments on samples from different rats. **P<0.01 vs fresh coronary arteries.