Effects of Ca2+ on the contractility and Sr2+ accumulation in the quail oviductal smooth muscle

There occur strong spontaneous contractions in the avian oviduct, which are considered to effect the locus of the ovum during ovum transport. Previously it has been shown that stretch regulates these contractions by means of some unknown mechanisms in this tissue. When a transversal strip, cut from the proximal magnum of the oviduct, was first kept at the length (L₀= length in situ) and then stretched to the length 200% L₀ the total integrated area of contractions increased 3.67±0.63 (6) times greater (p<0.01) than the contractility at the length L_o. Ca²⁺ concentration of 5 mM increased the total integrated area of the contractions 1.42±0.22 (7) greater than the control while the increase at 8 mM was only 1.17±0.30 (7). 0.5 mM Ca²⁺ decreased the contractility to 0.72±0.15 (7). In a 50 μM EGTA containing solution, the contractility decreased in a few minutes. Sr²⁺ used as an indirect tracer of Ca²⁺ accumulated both in control and stretched strips in the sarcoplasmic reticulum, the mitochondria, and the sarcolemma. The electron-dense deposits were, however, most clearly seen in the sarcolemma. Compared to the Sr²⁺ accumulating sites in erythrocytes, deposits were on the outer side of the cell membrane, thus suggesting the glycocalyx as an important site for Ca²⁺ exchange in this smooth muscle. The sarcolemmal invaginations, caveolae, accumulated Sr²⁺ similarly to other parts of the sarcolemma. During ovum transport, stretch changes the Ca²⁺ accessibility to muscle cells, and at structural level, this takes place in the sarcolemma.