EFFECTS OF 3,4-DIAMINOPYRIDINE AN MYOPLASMIC CALCIUM AND PHOSPHOINOSITIDE HYDROLYSIS IN FROG SARTORIUS MUSCLE-FIBERS

Using Ca2+-selective microelectrodes, the effects of 3, 4-diaminopyrid ine (DAP) on myoplasmic calcium ([Ca2+](i)) were studied in frog sator ius muscle fibers. DAP induced an increase of [Ca2+](i); in a concentr ation-dependent manner. DAP 1 mmol.L(-1) increased the [Ca2+](i) from control 166+/-41 nmol.L(-1) to 416+/-69 nmol.L(-1) (n=10). In the abse nce of external calcium, DAP still enhanced [Ca2+](i). [Ca2+](i) of th e fibers bathed in Ca2+-free Ringer's solution containing DAP 1 mmol.L (-1) was 152+/-43 nmol.L(-1) , which was significantly higher than 77/-35 nmol.L(-1) of [Ca2+](i) of the fibers in Ca2+-free Ringer's solut ion. In addition, DAP promoted the hydrolysis of phosphoinositides, an d DAP induced hydrolysis was more in the presence of external calcium. It is suggested that, through enhancing the hydrolysis of phosphoinos itides, DAP released Ca2+ from intracellular Ca2+ store in frog sartor ius muscle fibers.