1 By use of front-surface fluorometry with fura-2-loaded rabbit femoral arterial strips, both the cytosolic Ca2+ concentration ([Ca2+](i)) and force were simultaneously monitored. By utilizing the [Ca2+](i)-force curves, we were thus able to examine the temporal changes in the relationships between [Ca2+](i) and force ([Ca2+](i)-force relationship) during contractions induced by a high external K+ solution, noradrenaline (NA) and 5-hydroxytryptamine (5-HT). 2 The 'basic' [Ca2+](i)-force relationship of the Ca2+-induced contractions was obtained by the cumulative applications of extracellular Ca2+ (0-10 mM) during 118 mM K+-depolarization (Ca2+-contractions). 3 When each vascular strip was exposed to high external K+ (30 mM K+ -118 mM K+) solutions, the [Ca2+](i) abruptly increased until it reached a peak, and then slightly decreased and eventually reached a steady-state level. The force also rapidly rose to reach a maximum plateau level. The changes in [Ca2+](i) were more rapid than those in the force. Thus, the [Ca2+](i)-force curves observed during the contractions induced by high(+) (30 mM-118 mM) solutions showed a counter-clockwise rotation, over time. The entire curve shifted to the right, in a concentration-dependent manner, as compared with the line of the 'basic' [Ca2+](i)-force relationship of the Ca2+-contraction. However, the [Ca2+](i)-force relationship of the steady-state of contractions induced by the single dose applications of high K+ (30 mM-118 mM) overlapped with the line of the 'basic' [Ca2+](i)-force relationship of Ca2+-contractions. 4 As references, the levels of [Ca2+](i) and the force at rest (without stimulation) and at the steady-state of the contractions induced by a single dose application of 118 mM K+ solution were designated as 0% and 100%, respectively. When the vascular strips were exposed to NA (10(-5) M) and to 5-HT (10(-4) M), the [Ca2+](i) abruptly rose, and reached a peak (107.1 +/- 5.8% and 101.3 +/- 2.8%, respectively) after 1 min and 2 min, respectively (the [Ca2+](i)-rising phase), and thereafter declined with a similar time course (the [Ca2+](i)-declining phase) until reaching a low steady level (the steady-state phase). The force induced by 10(-5) M NA and 10(-4) M 5-HT reached a peak at 4 min (129%) and at 2 min (115%), respectively, and thereafter gradually declined. In contrast to the similarity in the [Ca2+](i) transient between NA and 5-HT, the force induced by NA declined more slowly and reached higher steady levels than that seen with 5-HT. The level of force 20 min after the application of NA and 5-HT was 112% and 72%, respectively. 5 In the entire time course of the 5-HT-induced contraction, i.e., in [Ca2+](i)-rising, [Ca2+](i)-declining and the steady-state phases, the [Ca2+](i)-force relation was almost the same as that of the Ca2+-contractions. In the [Ca2+](i)-rising phase of NA-induced contraction, the [Ca2+](i)-force relation was similar to that of the Ca2+-contractions. However, in the [Ca2+](i)-declining and the steady-state phases, NA produced a greater force than that expected from a given change in the [Ca2+](i) of the Ca2+-contractions, which resulted in a leftward shift of the [Ca2+](i)-force relation. The extent of the leftward shift depended on the concentration of NA. 6 These results suggest that (1) changes in [Ca2+](i) precede changes in the force during the high K+-induced contraction, (2) in the initial [Ca2+](i)-rising phase of the contractions induced by NA or by 5-HT, the [Ca2+](i)-force relation is similar to that of Ca2+-contractions, and (3) in the subsequent [Ca2+](i)-declining and the steady-state phases of the contractions, 5-HT demonstrated little enhancement in force for the given levels of [Ca2+](i), while NA induced a greater force for the given levels of[Ca2+](i), in the rabbit femoral artery. Based on the above findings we suggest the presence of a time-dependent and stimulus-specific modulation of the Ca2+ sensitivity in the contractile apparatus of arterial smooth muscles.
