Intracellular calcium transients recorded with Fura-2 in spontaneously active myocytes isolated from the atrioventricular node of the rabbit heart

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Abstract
We have used the fluorescent Ca indicator Fura-2 to assess the changes in intracellular calcium (Cai) in single spontaneously active myocytes isolated from the rabbit atrioventricular node (AVN). Simultaneous recordings of membrane potential and the Fura-2 ratio signal (which reflects Cai) showed that a transient rise of Cai occurred with each spontaneous action potential (AP). The AP upstroke preceded the rise in Cai and repolarization of the AP occurred faster than the decline of Cai. The level of Cai remained raised and progressively declined towards a baseline diastolic level during the subsequent pacemaker depolarization. The Fura-2 (Cai) transient in spontaneously active AVN cells had a time-to-peak of 49.2 +/- 5.4 ms (mean +/- s.e.m.; n = 7) and declined with a single exponential time course (time constant = 139.8 +/- 23.9 ms; n = 7). Application of 10 microM ryanodine completely and irreversibly abolished the Cai transient, identifying the sarcoplasmic reticulum (SR) as the major source of releasable Ca. Both removal of external Ca and block of L-type Ca channels (with 2 microM nifedipine) also abolished Cai transients, suggesting that Ca entry via L-type Ca-channels is involved in triggering the SR Ca release underlying the Cai transient. Removal of external Na (in the presence of 20 microM nifedipine to block L-type Ca channels) caused a reversible increase in Cai, showing that Na/Ca exchange is present in AVN cells and that it is involved in Cai regulation. Spontaneous Cai transients were abolished by 1 microM acetylcholine, and this was associated with a hyperpolarization of membrane potential and cessation of action potentials.(ABSTRACT TRUNCATED AT 250 WORDS)