Cytosolic calcium concentration‐force relation during contractions in the rabbit femoral artery: time‐dependency and stimulus specificity
Authors: Fukuizumi Yutaka , Kobayashi Sei , Nishimura Junji , Kanaide Hideo 
Published: July 2012

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).

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).

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.

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 10−5m NA and 10−4m 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.

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.

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.