Evidence for presence of Ca2+ channel-gated Ca2+ stores in neonatal human atrial myocytes

Abstract

The characteristics of Ca2+ signaling in fura 2-loaded whole cell-clamped myocytes obtained from samples of human atrial appendages of 3-day to 4-yr-old patients were examined. In isolated myocytes, activation of Ca2+ current (ICa) (2.47 +/- 0.23 pA/pF) at 0 mV elicited sizable intracellular Ca2+ (Cai) transients (240 +/- 45 nM), which were caused by the release of Ca2+ from intracellular stores as they were suppressed in the presence of ryanodine or caffeine. The voltage dependence of both Cai transients and ICa were similar and bell shaped. The rate of release of Ca2+, normalized for the maximal Ca2+ release, increased with age, indicating increased efficiency of Ca2+ signaling in more mature myocytes. The results suggest that ICa-gated release of Ca2+ from the sarcoplasmic reticulum is the primary mechanism regulating the signaling of contraction in early postnatal as well as older human atrial myocytes.

Ca2+ mobilization in fetal-human cardiac myocytes is stimulated by isoproterenol and inhibited by ryanodine

“…This is demonstrated by the significantly reduced duration of transients at 20% peak amplitude. This contrasts with the observation in 3-d-old human atrial myocytes (8) where no significant change in the decay of the [Ca 2+ ]i transient was observed following stimulation with isoproterenol, even though there was a significant increase in the inward calcium current and in the [Ca2+]i transient amplitude. There are three possible explanations for this discrepancy between neonatal-and fetal-human cardiac myocytes: (a) differences in the development of ventricular and atrial myocytes; (b) differences in culture methods (in the present study, myocytes were cultured for 4-5 d, whereas, atrial myocytes were examined within 1 h of tissue enzymatic digestion); or (c) differences in concentration of fluorescent probe [we used a fura-2 concentration of 3 ~tM, which has virtually no buffering effect on [Ca2*]i transients (19) and contrasts dramatically with the 200 ~tM fnra-2 used in atrial myocytes].…”

“…One consideration in using such tissue is the immaturity of mechanisms facilitating excitation-contraction (3,6,8). Recently, calcium mobilization during excitation-contraction was characterized in atrial myocytes from 3-d-old and 3-yr-old humans undergoing cardiac surgery (8). In this study, calcium-induced calcium release from the sarcoplasmic reticulum was the primary regulator of contraction signaling in early postnatal as well as older human atrial myocytes.…”

“…A few studies (4,5) have demonstrated that fetal heart tissue obtained from terminated pregnancies can provide a more consistent source of healthy human atrial and ventricle myocytes. One consideration in using such tissue is the immaturity of mechanisms facilitating excitation-contraction (3,6,8). Recently, calcium mobilization during excitation-contraction was characterized in atrial myocytes from 3-d-old and 3-yr-old humans undergoing cardiac surgery (8).…”

Developmental aspects of cardiac Ca2+signaling: interplay between RyR- and IP3R-gated Ca2+stores

“…Consistent with this idea, ryanodine (Ry) has been shown to diminish, but not eliminate, embryonic and neonatal cell contraction (36,45,51). Although functional RyRs are clearly present in postnatal cardiomyocytes (18), observations that inhibitors of SR function exert a more pronounced suppressive effect on cardiac action potential-mediated Ca 2ϩ transients in juvenile (88%) vs. neonatal (15%) cardiomyocytes indicate an increased reliance on and upregulation of RyR as a function of age (10,11). Antisense knockdown of IP 3 R 1 in embryonic stem cells markedly reduces cardiac cellular oscillations (35).…”

A new view of K+-induced contraction in rat aorta: the role of Ca2+ binding

“…After treatment, aliquots of cell suspension were mixed (1:1) with aliquots of ISS 1 containing CaCl 2 , 45 CaCl 2 and KCl (final concentrations: [Ca 2+ ] ex =20 mM and [ 45 Ca 2+ ] ex =1.5 mCi/ml, [K + ] ex =20 mM) (K20 ISS 1 , see Table 1). In some experiments, K20 ISS 1 contained ryanodine (5 mM), the inhibitor of Ca 2+ release from the sarcoplasmic reticulum [12], and oligomycin (10 6 g/ml), the metabolic inhibitor of Ca 2+ transport in mitochondria [17]. To explore the influence of Mg 2+ on 45 Ca 2+ uptake, aortic cells were treated with 20 mM K + in medium containing 5 mM Mg 2+ (K20Mg5 ISS 1 , Table 1).…”