FREE CYTOSOLIC CALCIUM DURING SPONTANEOUS CONTRACTIONS IN SMOOTH-MUSCLE OF THE GUINEA-PIG MESOTUBARIUM

The free intracellular calcium ion concentration ([Ca2+]i) was measured simultaneously with isometric force in strips of guinea-pig mesotubarium using the Fura-2 technique. During the relaxed period (5-15 min) between spontaneous contractions [Ca2+]i continues to decrease after full mechanical relaxation to reach a minimal level of 86±8 nM (n=9) just before the start of the next contraction. During the spontaneous contractions (5-15 min) [Ca2+]i reached a maximum of 211±19 nM and then oscillated between 155±16 nM and 194±9 nM. Increased extracellular Ca2+ concentration to 10 mM from the standard concentration of 1.5 mM caused a decreased frequency of spontaneous contractions and an increase in [Ca2+]i both in the relaxed and contracted states. In 10 mM extracellular Ca2+, addition of AlF4-, as 1 mM NaF + 10 μM AlCl3, caused a sustained increase in [Ca2+]i and maintained force. Addition of verapamil (10 μM) in this situation decreased [Ca2+]i to the resting level. The results suggest that the cyclic appearance of trains of action potentials is related to variation in [Ca2+]i, possibly via inactivation of Ca2+-dependent K+ channels. © 1990 Springer-Verlag.