title: Dependence of tonic tension on extracellular calcium in rat extraocular muscle.
abstract: Ca2+-free saline containing 3.0 mM Mg2+ virtually abolishes the tonic tension evoked by depolarization with a high K+ concentration of the tonic multiply innervated fibers of rat extraocular muscles. The tonic tension abolished by Ca2+ withdrawal is restored when Ca2+ is substituted by Sr2+ but not by Ni2+. The increase of Mg2+ reduces the tonic tension and displaces the tension-log K+ relationship to the right. Cd2+ significantly reduces the tension amplitude but does not shift the tension-log K+ relationship. The organic blocker of Ca2+ channels, nifedipine (1-10 microM), has no effect on the tonic tension. In contrast, diltiazem (20 microM) reduces the amplitude of the responses without changing the tension-log K+ relationship. Both foreign anions NO3- and SCN- potentiate tonic tension without changing the tension-log K+ relationship. SCN- increases the resting tension of the muscle; this effect depends on Ca2+. In conclusion, the disappearance of tonic tension after Ca2+ withdrawal is not due to depolarization of the fibers or inactivation of the contractile responses. It is suggested that entry of extracellular Ca2+, via a voltage-dependent Ca2+ conductance, or specific interactions of Ca2+ with membrane sites involved in the regulation of excitation-contraction coupling play a role in evoking tension in tonic fibers.
mesh_terms: D000818:Animals; D000838:Anions / Q000494:pharmacology; D002104:Cadmium / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002121:Calcium Channel Blockers / Q000494:pharmacology; D005110:Extracellular Space / Q000378:metabolism*; D008274:Magnesium / Q000494:pharmacology; D009119:Muscle Contraction; D009532:Nickel / Q000494:pharmacology; D009801:Oculomotor Muscles / Q000378:metabolism / Q000502:physiology*; D011188:Potassium / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D013324:Strontium / Q000494:pharmacology
chemical_list: D000838:Anions; D002121:Calcium Channel Blockers; D002104:Cadmium; D009532:Nickel; D008274:Magnesium; D011188:Potassium; D002118:Calcium; D013324:Strontium