Stretch induced tension rise in a molluscan smooth muscle skinned by freeze drying

Summary The anterior byssus retractor muscle (ABRM) ofMytilus edulis was skinned by freeze drying. Tension transients in response to quick length steps were recorded during isometric contraction induced in ATP salt solution containing 2×10^–6 M Ca²⁺. These transients consisted of four phases similar to those described by Huxley (1974) in skeletal muscle. Under certain conditions (stretch amplitude not larger than 0.6% L_O), and in particular in the presence of cyclic AMP, we observed a delayed tension rise following a quick stretch (stretch activation) which appears to be similar to the stretch activation of insect flight muscle (Jewell and Rüegg 1966).     

Lack of evidence for voltage dependent calcium channels on platelets

Intracellular calcium was measured in human platelets using the fluorescent calcium indicator Quin 2. A concentration dependent increase was observed with thrombin. Depolarisation induced by high KCl concentrations did not alter [Ca++]i. The calcium agonist Bay K 8644 did not affect resting levels or thrombin stimulated elevation of intracellular calcium. The calcium antagonists diltiazem, verapamil and PN 200-110 did not inhibit the thrombin stimulated elevation in [Ca++]i. Pretreatment of platelets with adenylate cyclase stimulants reduced the rate and magnitude of the maximal [Ca++]i elevation due to thrombin. In addition, thrombin stimulation of 45Ca++ influx was insensitive to Bay K 8644, verapamil, diltiazem and Pn 200-110. We conclude that functional voltage sensitive calcium channels are not present on human platelets.