Effects of extracellular ATP on freshly isolated mouse skeletal muscle cells during pre-natal and post-natal development

Extracellular adenosine 5'-triphosphate (ATP) has profound effects on membrane conductance and on the intracellular free [Ca2+] ([Ca2+](i)) in cultured skeletal muscle cells. The aim of the present study was to examine the occurrence and to characterize the properties of such responses during mammalian muscle development in vivo. The effect of ATP (0.2 mM) was tested on membrane current and [Ca2+](i) in freshly isolated pre- and post-natal mouse skeletal muscle cells. Pre-natal cells were from 14- to 19-day-old fetuses. In pre- and early post-natal cells, very small elevations of [Ca2+](i) (<50 nM) following ATP application could be detected with the fluorescent indicator fura-2. A clear subsarcolemmal rise in [Ca2+] was however associated to the presence of ATP, as demonstrated by increased activity of plasma membrane Ca2-activated K+ channels in cells bathed in a depolarizing, high-calcium-containing solution. In cells voltage-clamped at -80 mV in external Tyrode, ATP induced an inward current associated with an increased membrane conductance. The mean maximal amplitude of the ATP-induced current was -0.84+/-0.07 A/F (n=39). The response to ATP was still present after birth, although its amplitude tended to decrease with post-natal development and was completely absent in muscle cells from 3- to 6-month-old mice. The ATP-induced current could be abolished reversibly by suramin. Our results suggest that, over the range of developmental stages examined, skeletal muscle cells display an ionotropic purinergic signalling pathway with functional properties qualitatively consistent with what is observed in cultured myotubes.