"Voltage-activated Ca release" in rabbit, rat and guinea-pig cardiac myocytes, and modulation by internal cAMP.
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 degrees C. We used a train of conditioning pulses to ensure a standard SR load before each test pulse. In rabbit myocytes dialysed with 100 microM cAMP, 89.6 +/- 7.0% of the control intracellular Ca (Cai) 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 Cai 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 microM. In rat myocytes dialysed with 50 microM 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 microM. In guinea-pig myocytes dialysed with 100 microM cAMP, 74.6 +/- 3.6% of the control Cai 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 microM. 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 microM, VACR may play a role in triggering SR release. The role of VACR may increase under conditions which increase internal [cAMP].
Hobai
I A
IA
Department of Physiology, School of Medical Sciences, University of Bristol, Bristol BS8 1TD, UK.
Howarth
F C
FC
Pabbathi
V K
VK
Dalton
G R
GR
Hancox
J C
JC
Zhu
J Q
JQ
Howlett
S E
SE
Ferrier
G R
GR
Levi
A J
AJ
D000818:Animals; D002104:Cadmium / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects / Q000502:physiology; D002462:Cell Membrane / Q000378:metabolism; D000242:Cyclic AMP / Q000378:metabolism*; D004553:Electric Conductivity; D005456:Fluorescent Dyes; D006168:Guinea Pigs; D009206:Myocardium / Q000378:metabolism*; D009532:Nickel / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D011817:Rabbits; D051381:Rats; D013050:Spectrometry, Fluorescence
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D005456:Fluorescent Dyes; D002104:Cadmium; D009532:Nickel; D000242:Cyclic AMP; D002118:Calcium