Calcium transients caused by calcium entry are influenced by the sarcoplasmic reticulum in guinea-pig atrial myocytes.
1. Single atrial myocytes obtained by enzyme perfusion from hearts of adult guinea-pigs were investigated using whole-cell voltage clamp and Indo-1 micro-fluorometry. 2. In myocytes loaded with a solution containing citrate as a low-affinity, non-saturable Ca2+ chelator, two types of [Ca2+]i transients could be recorded during repetitive activation of L-type Ca2+ current. Both large and small [Ca2+]i transients occurred; large transients reached peak values of about 1 microM, and small transients were about 100 nM or less in amplitude. 3. In the case of the large transients, peak [Ca2+]i was usually reached with a variable delay after repolarization from a voltage step that activated calcium current (ICa). For the small transients the rise in [Ca2+]i paralleled ICa. Upon repolarization [Ca2+]i started to decay. 4. The small transients reflect entry of Ca2+ through Ca2+ channels (entry transients), whereas the large transients are due to entry and release from the sarcoplasmic reticulum (release transients). 5. The entry transients displayed a positive staircase pattern during trains of depolarizing voltage steps despite constant or even decreasing amplitude of ICa. The steepness of the staircase was increased by elevation of [Ca2+]o. Entry transients were always smallest immediately after a release transient. 6. After functional removal of the sarcoplasmic reticulum by caffeine (1-5 mM) the staircase pattern of the transients reflecting Ca2+ entry was abolished. 7. It is concluded that the staircase pattern is due to rapid uptake by the sarcoplasmic reticulum of Ca2+ entering the cell, resulting in an attenuation of the signal. The attenuation is strongest shortly after a release signal, when the rate of sequestration of Ca2+ by the SR should be highest. 8. Evidence is provided that a compartment of the SR is involved in attenuation of the entry transients. This compartment has been identified recently as a peripheral release compartment.
Lipp
P
P
Laboratory of Physiology, Catholic University Leuven, Belgium.
Pott
L
L
Callewaert
G
G
Carmeliet
E
E
D000818:Animals; D016275:Atrial Function; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000502:physiology; D003592:Cytophotometry; D006168:Guinea Pigs; D006321:Heart / Q000187:drug effects / Q000502:physiology; D009206:Myocardium / Q000166:cytology / Q000378:metabolism*; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism*
D015220:Calcium Channels; D002110:Caffeine; D002118:Calcium