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Cross‐coupling between voltage‐dependent Ca 2+ channels and ryanodine receptors in developing ascidian muscle blastomeres
Author(s) -
Nakajo Koichi,
Chen Ling,
Okamura Yasushi
Publication year - 1999
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1111/j.1469-7793.1999.695ab.x
Subject(s) - ryanodine receptor , cyclopiazonic acid , endoplasmic reticulum , depolarization , thapsigargin , chemistry , biophysics , voltage dependent calcium channel , calcium , caffeine , biochemistry , biology , endocrinology , organic chemistry
1 Ascidian blastomeres of muscle lineage express voltage‐dependent calcium channels (VDCCs) despite isolation and cleavage arrest. Taking advantage of these large developing cells, developmental changes in functional relations between VDCC currents and intracellular Ca 2+ stores were studied. 2 Inactivation of ascidian VDCCs is Ca 2+ dependent, as demonstrated by two pieces of evidence: (1) a bell‐shaped relationship between prepulse voltage and amplitude during the test pulse in Ca 2+ , but not in Ba 2+ , and (2) the decay kinetics of Ca 2+ currents ( I Ca ) obtained as the size of tail currents. 3 During replacement in the external solution of Ca 2+ with Ba 2+ , the inward current appeared biphasic: it showed rapid decay followed by recovery and slow decay. This current profile was most evident in the mixed bath solution (2% Ca 2+ and 98% Ba 2+ , abbreviated to ‘2Ca/98Ba’). 4 The biphasic profile of I 2Ca/98Ba was significantly attenuated in caffeine and in ryanodine, indicating that Ca 2+ release is involved in shaping the current kinetics of VDCCs. After washing out the caffeine, the biphasic pattern was reproducibly restored by depolarizing the membrane in calcium‐rich solution, which is expected to refill the internal Ca 2+ stores. 5 The inhibitors of endoplasmic reticulum (ER) Ca 2+ ‐ATPase (SERCAs) cyclopiazonic acid (CPA) and thapsigargin facilitated elimination of the biphasic profile with repetitive depolarization. 6 At a stage earlier than 36 h after fertilization, the biphasic profile of I 2Ca/98Ba was not observed. However, caffeine induced a remarkable decrease in the amplitude of I 2Ca/98Ba and this suppression was blocked by microinjection of the Ca 2+ chelator BAPTA, showing the presence of caffeine‐sensitive Ca 2+ stores at this stage. 7 Electron microscopic observation shows that sarcoplasmic membranes (SR) arrange closer to the sarcolemma with maturation, suggesting that the formation of the ultrastructural machinery underlies development of the cross‐coupling between VDCCs and Ca 2+ stores.