ABSTRACT
We tested the hypothesis that eccentric contractions activate mechanosensitive or stretch-activated ion channels (SAC) in skeletal muscles, producing increased cation conductance. Resting membrane potentials and contractile function were measured in rat tibialis anterior muscles after single or multiple exposures to a series of eccentric contractions. Each exposure produced a significant and prolonged (>24 h) membrane depolarization in exercised muscle fibers. The magnitude and duration of the depolarization were related to the number of contractions. Membrane depolarization was due primarily to an increase in Na(+) influx, because the estimated Na(+)-to-K(+) permeability ratio was increased in exercised muscles and resting membrane potentials could be partially repolarized by substituting an impermeant cation for extracellular Na(+) concentration. Neither the Na(+)/H(+) antiport inhibitor amiloride nor the fast Na(+) channel blocker TTX had a significant effect on the depolarization. In contrast, addition of either of two nonselective SAC inhibitors, streptomycin or Gd(3+), produced significant membrane repolarization. The results suggest that muscle fibers experience prolonged depolarization after eccentric contractions due, principally, to the activation of Na(+)-selective SAC.
Subject(s)
Ion Channels/metabolism , Mechanoreceptors/metabolism , Muscle Contraction/physiology , Muscle, Skeletal/physiology , Physical Conditioning, Animal/physiology , Amiloride/pharmacology , Animals , Cations/metabolism , Cell Membrane Permeability/drug effects , Female , Gadolinium/pharmacology , Ion Channels/antagonists & inhibitors , Meglumine/metabolism , Membrane Potentials/drug effects , Muscle Contraction/drug effects , Muscle Fibers, Skeletal/drug effects , Muscle Fibers, Skeletal/physiology , Muscle, Skeletal/cytology , Muscle, Skeletal/drug effects , Muscle, Skeletal/injuries , Potassium/metabolism , Rats , Rats, Sprague-Dawley , Sodium/metabolism , Sodium Channel Blockers , Sodium Channels/metabolism , Sodium-Hydrogen Exchangers/antagonists & inhibitors , Sodium-Hydrogen Exchangers/metabolism , Streptomycin/pharmacology , Tetrodotoxin/pharmacologySubject(s)
Follicle Stimulating Hormone/pharmacology , Gonadal Steroid Hormones/metabolism , Luteinizing Hormone/pharmacology , Ovarian Follicle/metabolism , Progesterone/metabolism , Animals , Dose-Response Relationship, Drug , Estradiol/metabolism , Female , In Vitro Techniques , Kinetics , Ovarian Follicle/drug effects , Rabbits , Testosterone/metabolismABSTRACT
Preovulatory follicles were dissected from ovaries of estrous rabbits. Four follicles were placed in each chamber of a perifusion system. Medium 199, containing 10% rabbit serum and antibiotics, eluted through the chambers at a rate of 1 ml/10 min. Every 6 h for 36 h, 1 microgram of LH and 50 ng of FSH were injected into each chamber. P4, T and E2 were measured in the medium. P4 exhibited a peak after each of the injections at 6, 12, 18, 24 and 30 h. E2 and T gave major peaks after the injection at 0 time, with minor peaks after the injection at 6 h; thereafter T and E2 secretion gradually decreased to levels below those of baseline.
