Home | Help | Feedback | Subscriptions | Archive | Search | Table of Contents

This Article Full Text Full Text (PDF, 403K) PPT slides of all figures Alert me when this article is cited Citation Map Services Email this article Similar articles in this journal Similar articles in PubMed Alert me to new content in the JGP Download to citation manager Citing Articles Citing Articles via CrossRef Citing Articles via Google Scholar Google Scholar Articles by Narita, K. Articles by Kuba, K. Search for Related Content PubMed PubMed Citation Articles by Narita, K. Articles by Kuba, K. Pubmed/NCBI databases Compound via MeSH Substance via MeSH Hazardous Substances DB CALCIUM COMPOUNDS CALCIUM, ELEMENTAL GALLIC ACID MAGNESIUM COMPOUNDS MAGNESIUM, ELEMENTAL Social Bookmarking                            What's this?

Article

K. Narita, T. Akita^*, M. Osanai^||, T. Shirasaki^,¶, H. Kijima^**, and K. Kuba^*,

From the ^* Department of Physiology, School of Medicine, Nagoya University, Showa-ku, Nagoya 466-8550, Japan; Department of Physiology, Saga Medical School, Saga 849-8501, Japan; Department of Physiology, Kawasaki Medical School, Kurashiki 701-0192, Japan; ^|| Department of Pharmacology, Tokyo Medical and Dental University, School of Medicine, Tokyo 113-8519, Japan; ^¶ Department of Physiology, Kansai Medical University, Moriguchi 570-0074, Japan; and ^** Department of Physics, School of Science, Nagoya University, Nagoya 464-8602, Japan

The extent to which Ca²⁺-induced Ca²⁺ release (CICR) affects transmitter release is unknown. Continuous nerve stimulation (20–50 Hz) caused slow transient increases in miniature end-plate potential (MEPP) frequency (MEPP-hump) and intracellular free Ca²⁺ ([Ca²⁺]_i) in presynaptic terminals (Ca²⁺-hump) in frog skeletal muscles over a period of minutes in a low Ca²⁺, high Mg²⁺ solution. Mn²⁺ quenched Indo-1 and Fura-2 fluorescence, thus indicating that stimulation was accompanied by opening of voltage-dependent Ca²⁺ channels. MEPP-hump depended on extracellular Ca²⁺ (0.05–0.2 mM) and stimulation frequency. Both the Ca²⁺- and MEPP-humps were blocked by 8-(N,N-diethylamino)octyl3,4,5-trimethoxybenzoate hydrochloride (TMB-8), ryanodine, and thapsigargin, but enhanced by CN^–. Thus, Ca²⁺-hump is generated by the activation of CICR via ryanodine receptors by Ca²⁺ entry, producing MEPP-hump. A short interruption of tetanus (<1 min) during MEPP-hump quickly reduced MEPP frequency to a level attained under the effect of TMB-8 or thapsigargin, while resuming tetanus swiftly raised MEPP frequency to the previous or higher level. Thus, the steady/equilibrium condition balancing CICR and Ca²⁺ clearance occurs in nerve terminals with slow changes toward a greater activation of CICR (priming) during the rising phase of MEPP-hump and toward a smaller activation during the decay phase. A short pause applied after the end of MEPP- or Ca²⁺-hump affected little MEPP frequency or [Ca²⁺]_i, but caused a quick increase (faster than MEPP- or Ca²⁺-hump) after the pause, whose magnitude increased with an increase in pause duration (<1 min), suggesting that Ca²⁺ entry-dependent inactivation, but not depriming process, explains the decay of the humps. The depriming process was seen by giving a much longer pause (>1 min). Thus, ryanodine receptors in frog motor nerve terminals are endowed with Ca²⁺ entry-dependent slow priming and fast inactivation mechanisms, as well as Ca²⁺ entry-dependent activation, and involved in asynchronous exocytosis. Physiological significance of CICR in presynaptic terminals was discussed.

Key Words: intracellular calcium • Ca²⁺ influx • ryanodine receptor • transmitter release

Abbreviations: CICR, Ca²⁺-induced Ca²⁺ release; IP₃, inositoltrisphosphate; MEPP, miniature end-plate potential; TMB-8, 8-(N,N-diethylamino)octyl3,4,5-trimethoxybenzoate hydrochloride

CiteULike    Complore    Connotea    Del.icio.us    Digg    Facebook    Reddit    Technorati    Twitter    What's this?

Home | Help | Feedback | Subscriptions | Archive | Search | Table of Contents