'Voltage-activated Ca release' in rabbit, rat and guinea-pig cardiac myocytes, and modulation by internal cAMP

Ion A. Hobai, F. Chris Howarth, Vijay K. Pabbathi, Geoff R. Dalton, Jules C. Hancox, Jie Quan Zhu, Susan E. Howlett, Gregory R. Ferrier, Allan J. Levi

Research output: Contribution to journalArticlepeer-review

61 Citations (Scopus)

Abstract

It is widely believed that Ca release from the sarcoplasmic reticulum (SR) in heart muscle is due to 'Ca-induced Ca-release' (CICR), triggered by transmembrane Ca entry. However, in intact guinea-pig cells or cells dialysed with cAMP there may be an additional mechanism - SR release may be activated directly by membrane depolarisation without Ca entry. The first objective of the present study was to investigate whether this 'voltage-activated Ca release' (VACR) mechanism is present across species such as rabbit, rat and guinea-pig. The second objective was to characterise the dependence of a VACR mechanism on internal [cAMP]. Membrane current was measured with the whole-cell patch clamp technique, intracellular [Ca] was monitored with Fura-2 (or a combination of Fluo-3/SNARF-1). Rapid changes of superfusate (within 100 ms) were made using a system which maintained cell temperature at 37°C. We used a train of conditioning pulses to ensure a standard SR load before each test pulse. In rabbit myocytes dialysed with 100 μM cAMP, 89.6 ± 7.0% of the control intracellular Ca (Ca(i)) transient was still elicited by depolarisation during a switch to 5 mM Ni, which blocked pathways for Ca entry. This suggested that rabbit myocytes possess a VACR mechanism. The percentage of control Ca(i) transient elicited by depolarisation in the presence of 5 mM Ni (i.e. magnitude of VACR) increased in a graded fashion with the pipette [cAMP] between zero and 100 μM. In rat myocytes dialysed with 50 μM cAMP, 64.4 ± 6.2% of SR release was activated by depolarisation in the presence of 5 mM Ni, suggesting the presence of a VACR mechanism. The extent to which VACR triggered SR release increased with the pipette [cAMP] between zero and 50 μM. In guinea-pig myocytes dialysed with 100 μM cAMP, 74.6 ± 3.6% of the control Ca(i) transient was elicited by depolarisation in the presence of 5 mM Ni. The degree to which VACR triggered SR release was also graded with the pipette [cAMP] between zero and 100 μM. It therefore appears that each of the three species might possess a VACR mechanism which can be modulated by the internal [cAMP]. This may reflect an effect of cAMP to phosphorylate key proteins involved in excitation-contraction coupling. Under normal physiological conditions with a basal [cAMP] between 2 and 20 μM, VACR may play a role in triggering SR release. The role of VACR may increase under conditions which increase internal [cAMP].

Original languageEnglish
Pages (from-to)164-173
Number of pages10
JournalPflugers Archiv European Journal of Physiology
Volume435
Issue number1
DOIs
Publication statusPublished - 1997
Externally publishedYes

Keywords

  • Excitation-contraction coupling
  • L-type Ca current
  • Ryanodine receptor
  • Sarcoplasmic reticulum
  • Voltage sensor
  • Voltage-activated calcium release (VACR)

ASJC Scopus subject areas

  • Physiology
  • Clinical Biochemistry
  • Physiology (medical)

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