RESTING MEMBRANE-PROPERTIES OF LOCUST MUSCLE AND THEIR MODULATION II - ACTIONS OF THE BIOGENIC-AMINE OCTOPAMINE

Ionic currents in the resting membrane of locust jumping muscle and th eir modulation by the biogenic amine octopamine were investigated usin g the two-electrode voltage clamp. A Cl- conductance, G(Cl,H), which S lowly activates on hyperpolarization, can be induced by raising the in tracellular Cl- concentration via diffusion of Cl- ions from the recor ding electrode. The instantaneous I-V characteristic of the current, I -Cl,I-H, is linear and reverses at the same potential as the gamma-ami nobutyric acid (GABA)-mediated Cl- current. Elevation of [Cl-](i) incr eases the maximal steady state G(Cl,H) (G(max)) and shifts the activat ion curve of G(Cl,H) to more positive potentials. Octopamine enhances G(Cl,H), mainly by increasing G(max). Octopamine also lowers the resti ng K+ conductance (G(K,r)) It reduces a hyperpolarization-activated co mponent (G(K,H)) of G(K,r), mainly by decreasing G(max). Octopamine al so transiently stimulates the Na+/K+ pump although this effect was not always seen. The effects of octopamine on;he Cl- and K+ conductances are mimicked by membrane permeant cyclic nucleotides. The modulation o f G(K,r), but not that of G(Cl,H), seems to be mediated by protein kin ase A (PKA). PKA seems to be constitutively activated as indicated by the pronounced increase in GK,r induced by a PKA inhibitor, H89. The p roperties of G(Cl,H) and related Cl- conductances in invertebrate and vertebrate neurons are compared. G(Cl,H) probably supports efflux of C l- ions accumulating in the fibers during synaptic inhibition. Octopam ine's multiple modulation at the level of the muscle cell membrane, in conjunction with previously established effects on synaptic transmiss ion and excitation-contraction coupling, are suited to support strong and rapid muscle contractions.