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Intracellular calcium and Na + ‐Ca 2+ exchange current in isolated toad pacemaker cells
Author(s) -
Ju YueKun,
Allen David G.
Publication year - 1998
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.1469-7793.1998.153br.x
Subject(s) - calcium , diastole , extracellular , bapta , medicine , chemistry , ryanodine receptor , intracellular , fura 2 , biophysics , toad , calcium in biology , endoplasmic reticulum , egta , endocrinology , blood pressure , biology , biochemistry , cytosol , enzyme
1 Single pacemaker cells were isolated from the sinus venosus of cane toad ( Bufo marinus ) in order to study the mechanisms involved in the spontaneous firing rate of action potentials. Intracellular calcium concentration ([Ca 2+ ] i ) was measured with indo‐1 to determine whether [Ca 2+ ] i influenced firing rate. A rapid transient rise of [Ca 2+ ] i was recorded together with each spontaneous action potential. [Ca 2+ ] i at the peak of systole was 655 ± 64 nM and the minimum at the end of diastole was 195 ± 15 nM. 2 Reduction of extracellular Ca 2+ concentration from 2 to 0.5 mM caused a reduction in both systolic and diastolic [Ca 2+ ] i and the spontaneous firing rate also gradually declined. 3 Application of the acetoxymethyl (AM) ester of BAPTA (10 μM), in order to increase intracellular calcium buffering, caused a decline in systolic and diastolic [Ca 2+ ] i . The firing rate declined progressively until the cells stopped firing after 10‐15 min. At the time that firing ceased, the diastolic [Ca 2+ ] i had declined by 141 ± 38 nM. 4 In the presence of ryanodine (2 μM), which interferes with Ca 2+ release from the sarcoplasmic reticulum, the systolic and diastolic [Ca 2+ ] i both declined and the firing rate decreased until the cells stopped firing. At quiescence diastolic [Ca 2+ ] i had declined by 93 ± 20 nM. 5 Exposure of the cells to Na + ‐free solution caused a rise in [Ca 2+ ] i which exceeded the systolic level after 4.8 ± 0.3 s. This rise is consistent with Ca 2+ entry on a Na + ‐Ca 2+ exchanger. 6 Rapid application of caffeine (10‐20 mM) to cells clamped at ‐60 mV caused a rapid increase in [Ca 2+ ] i which then spontaneously declined. An inward current with a similar time course to that of [Ca 2+ ] i was also generated. Application of Ni 2+ (5 mM) or 2,4‐dichlorobenzamil (25 μM) reduced the amplitude of the inward current produced by caffeine by 96 ± 1 % and 74 ± 10 %, respectively. In a Na + ‐free solution the caffeine‐induced current was reduced by 93 ± 7 %. 7 Under a variety of circumstances the diastolic [Ca 2+ ] i showed a close association with pacemaker firing rate. The existence of a Na + ‐Ca 2+ exchanger and its estimated contribution to inward current during the pacemaker potential suggest that the Na + ‐Ca 2+ exchange current makes a contribution to pacemaker activity.