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The mechanisms of sarcoplasmic reticulum Ca 2+ release in toad pacemaker cells
Author(s) -
Ju YueKun,
Allen David G.
Publication year - 2000
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.1469-7793.2000.t01-1-00695.x
Subject(s) - depolarization , nifedipine , biophysics , chemistry , tetrodotoxin , toad , membrane potential , endoplasmic reticulum , cardiac transient outward potassium current , voltage clamp , extracellular , medicine , calcium , electrophysiology , patch clamp , biology , biochemistry , organic chemistry
1 The mechanisms of sarcoplasmic reticulum (SR) Ca 2+ release in pacemaker cells from the sinus venosus of the cane toad ( Bufo marinus ) were studied. Single, isolated cells were voltage clamped using a nystatin‐perforated patch. Ionic currents and intracellular Ca 2+ concentration ([Ca 2+ ] i ) were recorded simultaneously. 2 Depolarizations of 300 ms duration from a holding potential of −55 mV produced an inward current which had a bell‐shaped relationship with voltage. Inward current first appeared at about −45 mV, reached a maximum of −343 ± 46 pA at −15 mV and reversed at +45 mV. In contrast the amplitude of the increase in [Ca 2+ ] i caused by depolarization (Ca 2+ transient) increased monotonically with the increasing depolarization. At −15 mV the amplitude of the Ca 2+ transient was 243 ± 33 n m and at +45 mV it was 411 ± 43 n m . 3 The inward current produced by depolarizations to −5 mV was largely eliminated by the L‐type Ca 2+ channel blocker nifedipine (10 μM) while 37 ± 7% of the Ca 2+ transient persisted. A significantly larger proportion of the Ca 2+ transient (56 ± 5%) remained at +85 mV in the presence of nifedipine. 4 The SR Ca 2+ pump inhibitor 2,5‐di( tert ‐butyl)‐1,4‐hydroquinone (10 μ m ), which causes depletion of the SR Ca 2+ , reduced the amplitude of the Ca 2+ transient to 34 ± 1% of control, irrespective of the voltage. 5 Brief exposure to extracellular Ca 2+ ‐free solution abolished the Ca 2+ transients caused by depolarization while the caffeine‐induced Ca 2+ release persisted. 6 Tetrodotoxin (1 μ m ) had no effect on the amplitude of the depolarization‐induced Ca 2+ transient, although it reduced the fast component of the inward current. In contrast, Ni 2+ (5 m m ) abolished the Ca 2+ transients at any given voltage. Ni 2+ also abolished spontaneous Ca 2+ transients. 7 In conclusion, in toad pacemaker cells Ca 2+ release from SR contributes approximately 66% of the Ca 2+ involved in the Ca 2+ transient and requires extracellular Ca 2+ influx to trigger its release. The L‐type Ca 2+ channels and Na + ‐Ca 2+ exchange are major sources of Ca 2+ influx under physiological conditions.