Resting Membrane Properties of Locust Muscle and Their Modulation I. Actions of the Neuropeptides YGGFMRFamide and Proctolin

Abstract

Walther, Christian, Klaus E. Zittlau, Harald Murck, and Karlheinz Voigt. Resting membrane properties of locust muscle and their modulation. I. Actions of the neuropeptides YGGFMRFamide and proctolin. J. Neurophysiol. 80: 771–784, 1998. The resting K+ conductance ( G K,r) of locust jumping muscle and its modulation by two neuropeptides, proctolin (Arg-Tyr-Leu-Pro-Thr) and YGGFMRFamide (Tyr-Gly-Gly-Phe-Met-Arg-Phe-NH2), were investigated using the two-electrode voltage clamp. At a physiological [K+]o of 10 mM, G K,r accounts for ∼90% of the membrane resting conductance, and the resting membrane potential differs by ≤1 mV from E K (mean: −74 mV). There is a K+ conductance that slowly activates on hyperpolarization ( G K,H) and that seems to be largely located in the transverse tubules. Steady-state activation of G K,H was analyzed by tail current measurements. G K,H is activated partially at E K but accounts for probably ≤50% of total resting K+ conductance. Raising [K+]o caused a large increase in G K,r and in maximal steady state G K,H without shifting the voltage sensitivity of G K,H. YGGFMRFamide and proctolin reduce G K,H, mainly affecting the maximal steady-state conductance. The voltage-insensitive component of the resting K+ conductance is also reduced. The conductance suppressed by the peptides exhibited an outwardly rectifying instantaneous current/voltage-characteristic that is quite similar to that of G K,H. The actions of the two peptides appeared to be identical, but proctolin was by some two orders of magnitude more potent than YGGFMRFamide. The effects of both peptides are mediated by G proteins. They are mimicked by phorbol esters but do not seem to be initiated by either branch of the phospholipase C-dependent intracellular pathways. The properties of the resting K+ conductance in locust muscle and other invertebrate muscles are compared. The biological significance of peptide-induced reduction in resting K+ conductance is discussed in view of the known property of proctolin to support tonic force as opposed to FMRFamide-peptides that support quick leg movements.