beta-Adrenergic stimulation induces acetylcholine to activate ATP-sensitive K+ current in cat atrial myocytes

Multiresolution analysis of pathological changes in cerebral venous dynamics in newborn mice with intracranial hemorrhage: adrenorelated vasorelaxation

Intracranial hemorrhage (ICH) is the major problem of modern neonatal intensive care. Abnormalities of cerebral venous blood flow (CVBF) can play a crucial role in the development of ICH in infants. The mechanisms underlying these pathological processes remain unclear; however it has been established that the activation of the adrenorelated vasorelaxation can be an important reason. Aiming to reach a better understanding of how the adrenodependent relaxation of cerebral veins contributes to the development of ICH in newborns, we study here the effects of pharmacological stimulation of adrenorelated dilation of the sagittal sinus by isoproterenol on the cerebral venous hemodynamics. Our study is performed in newborn mice at different stages of ICH using the laser speckle contrast imaging and wavelet analysis of the vascular dynamics of CVBF. We show that the dilation of the sagittal sinus with the decreased velocity of blood flow presides to the stress-induced ICH in newborn mice. These morphofunctional vascular changes are accompanied by an increased variance of the wavelet-coefficients in the areas of endothelial and non-endothelial (KATP-channels activity of vascular muscle) sympathetic components of the CVBF variability. Changes in the cerebral venous hemodynamics at the latent stage of ICH are associated with a high responsiveness of the sagittal sinus to isoproterenol quantifying by wavelet-coefficients related to a very slow region of the frequency domain. The obtained results certify that a high activation of the adrenergic-related vasodilatory responses to severe stress in newborn mice can be one of the important mechanisms underlying the development of ICH. Thus, the venous insufficiency with the decreased blood outflow from the brain associated with changes in the endothelial and the sympathetic components of CVBF-variability can be treated as prognostic criteria for the risk of ICH during the first days after birth.

Electrophysiological effects of haloperidol on isolated rabbit Purkinje fibers and guinea pigs papillary muscles under normal and simulated ischemia

AIM

Overdoses of haloperidol are associated with major ventricular arrhythmias, cardiac conduction block, and sudden death. The aim of this experiment was to study the effect of haloperidol on the action potentials in cardiac Purkinje fibers and papillary muscles under normal and simulated ischemia conditions in rabbits and guinea pigs.

METHODS

Using the standard intracellular microelectrode technique, we examined the effects of haloperidol on the action potential parameters [action potential amplitude (APA), phase 0 maximum upstroke velocity (V(max)), action potential amplitude at 90% of repolarization (APD(90)), and effective refractory period (ERP)] in rabbit cardiac Purkinje fibers and guinea pig cardiac papillary cells, in which both tissues were under simulated ischemic conditions.

RESULTS

Under ischemic conditions, different concentrations of haloperidol depressed APA and prolonged APD(90) in a concentration-dependent manner in rabbit Purkinje fibers. Haloperidol (3 micromol/L) significantly depressed APA and prolonged APD(90), and from 1 micromol/L, haloperidol showed significant depression on V(max); ERP was not significantly affected. In guinea pig cardiac papillary muscles, the thresholds of significant reduction in APA, V(max), EPR, and APD(90) were 10, 0.3, 1, and 1 mumol/L, respectively, for haloperidol.

CONCLUSION

Compared with cardiac conductive tissues, papillary muscles were more sensitive to ischemic conditions. Under ischemia, haloperidol prolonged ERP and APD(90) in a concentration-dependent manner and precipitated the decrease in V(max) induced by ischemia. The shortening of ERP and APD(90) in papillary muscle action potentials may be inhibited by haloperidol.

Characterization of a hyperpolarization-activated time-dependent potassium current in canine cardiomyocytes from pulmonary vein myocardial sleeves and left atrium

Cardiomyocytes from the pulmonary vein sleeves (PVs) are known to play an important role in atrial fibrillation. PVs have been shown to exhibit time-dependent hyperpolarization-induced inward currents of uncertain nature. We observed a time-dependent K(+) current upon hyperpolarization of PV and left atrial (LA) cardiomyocytes (I(KH)) and characterized its biophysical and pharmacological properties. The activation time constant was weakly voltage dependent, ranging from 386 +/- 14 to 427 +/- 37 ms between -120 and -90 mV, and the half-activation voltage averaged -93 +/- 4 mV. I(KH) was larger in PV than LA cells (e.g. at -120 mV: -2.8 +/- 0.3 versus-1.9 +/- 0.2 pA pF(-1), respectively, P < 0.01). The reversal potential was approximately -84 mV with 5.4 mm[K(+)](o) and changed by 55.7 +/- 2.4 mV per decade [K(+)](o) change. I(KH) was exquisitely Ba(2+) sensitive, with a 50% inhibitory concentration (IC(50)) of 2.0 +/- 0.3 microm (versus 76.0 +/- 17.9 microm for instantaneous inward-rectifier current, P < 0.01), and showed similar Cs(+) sensitivity to instantaneous current. I(KH) was potently blocked by tertiapin-Q, a selective Kir3-subunit channel blocker (IC(50) 10.0 +/- 2.1 nm), was unaffected by atropine and was significantly increased by isoproterenol (isoprenaline), carbachol and the non-hydrolysable guanosine triphosphate analogue GTPgammaS. I(KH) activation by carbachol required GTP in the pipette and was prevented by pertussis toxin pretreatment. Tertiapin-Q delayed repolarization in atropine-exposed multicellular atrial preparations studied with standard microelectrodes (action potential duration pre- versus post-tertiapin-Q: 190.4 +/- 4.3 versus 234.2 +/- 9.9 ms, PV; 202.6 +/- 2.6 versus 242.7 +/- 6.2 ms, LA; 2 Hz, P < 0.05 each). Seven-day atrial tachypacing significantly increased I(KH) (e.g. at -120 mV in PV: from -2.8 +/- 0.3 to -4.5 +/- 0.5 pA pF(-1), P < 0.01). We conclude that I(KH) is a time-dependent, hyperpolarization-activated K(+) current that likely involves Kir3 subunits and appears to play a significant role in atrial physiology.

Transfection of a phosphatidyl-4-phosphate 5-kinase gene into rat atrial myocytes removes inhibition of GIRK current by endothelin and alpha-adrenergic agonists

GIRK (G protein-activated inward-rectifying K(+) channel) channels, important regulators of membrane excitability in the heart and in the central nervous, are activated by interaction with betagamma subunits from heterotrimeric G proteins upon receptor stimulation. For activation interaction of the channel with phosphatidylinositol 4,5-bisphosphate (PtIns(4,5)P(2)) is conditional. Previous studies have provided evidence that in myocytes PtIns(4,5)P(2) levels relevant to GIRK channel regulation are under regulatory control of receptors activating phospholipase C. In the present study a phosphatidyl-4-phosphate 5-kinase was expressed in atrial myocytes by transient transfection. This did not affect basal properties of GIRK current activated by acetylcholine via M(2) receptors but completely abolished inhibition of guanosine triphosphate-gamma-S activated current by endothelin-1 or alpha-adrenergic agonists. We conclude that though PtIns(4,5)P(2) is conditional for channel gating, its normal level in the membrane is not limiting basal function of GIRK channels. Moreover, our data provide further evidence for a regulation of GIRK channels by alpha(1A) receptors and endothelin-A receptors, endogenously expressed in atrial myocytes, via depletion of PtIns(4,5)P(2).

Beta 2-adrenergic receptor signaling acts via NO release to mediate ACh-induced activation of ATP-sensitive K+ current in cat atrial myocytes

In atrial myocytes, an initial exposure to isoproterenol (ISO) acts via cAMP to mediate a subsequent acetylcholine (ACh)-induced activation of ATP-sensitive K(+) current (I(K,ATP)). In addition, beta-adrenergic receptor (beta-AR) stimulation activates nitric oxide (NO) release. The present study determined whether the conditioning effect of beta-AR stimulation acts via beta(1)- and/or beta(2)-ARs and whether it is mediated via NO signaling. 0.1 microM ISO plus ICI 118,551 (ISO-beta(1)-AR stimulation) or ISO plus atenolol (ISO-beta(2)-AR stimulation) both increased L-type Ca(2+) current (I(Ca,L)) markedly, but only ISO-beta(2)-AR stimulation mediated ACh-induced activation of I(K,ATP). 1 microM zinterol (beta(2)-AR agonist) also increased I(Ca,L) and mediated ACh-activated I(K,ATP). Inhibition of NO synthase (10 microM L-NIO), guanylate cyclase (10 microM ODQ), or cAMP-PKA (50 microM Rp-cAMPs) attenuated zinterol-induced stimulation of I(Ca,L) and abolished ACh-activated I(K,ATP). Spermine-NO (100 microM; an NO donor) mimicked beta(2)-AR stimulation, and its effects were abolished by Rp-cAMPs. Intracellular dialysis of 20 microM protein kinase inhibitory peptide (PKI) abolished zinterol-induced stimulation of I(Ca,L). Measurements of intracellular NO ([NO](i)) using the fluorescent indicator DAF-2 showed that ISO-beta(2)-AR stimulation or zinterol increased [NO](i). L-NIO (10 microM) blocked ISO- and zinterol-induced increases in [NO](i). ISO-beta(1)-AR stimulation failed to increase [NO](i). Inhibition of G(i)-protein by pertussis toxin significantly inhibited zinterol-mediated increases in [NO](i). Wortmannin (0.2 microM) or LY294002 (10 microM), inhibitors of phosphatidylinositol 3'-kinase (PI-3K), abolished the effects of zinterol to both mediate ACh-activated I(K,ATP) and stimulate [NO](i). We conclude that both beta(1)- and beta(2)-ARs stimulate cAMP. beta(2)-ARs act via two signaling pathways to stimulate cAMP, one of which is mediated via G(i)-protein and PI-3K coupled to NO-cGMP signaling. Only beta(2)-ARs acting exclusively via NO signaling mediate ACh-induced activation of I(K,ATP). NO signaling also contributes to beta(2)-AR stimulation of I(Ca,L). The differential effects of beta(1)- and beta(2)-ARs can be explained by the coupling of these two beta-ARs to different effector signaling pathways.

Overexpression of beta 1 and beta 2 adrenergic receptors in rat atrial myocytes. Differential coupling to G protein-gated inward rectifier K(+) channels via G(s) and G(i)/o

G protein-activated inwardly rectifying K(+) (GIRK) channels, expressed in atrial myocytes, various neurons, and endocrine cells, represent the paradigmatic target of beta gamma subunits released from activated heterotrimeric G proteins. These channels contribute to physiological slowing of cardiac frequency and synaptic inhibition. They are activated by beta gamma dimers released upon stimulation of receptors coupled to pertussis toxin-sensitive G proteins (G(i/o)), whereas beta gamma released from G(s) do not converge on the channel subunits. This is in conflict with the finding that dimeric combinations of various beta and gamma subunits can activate GIRK channels with little specificity. In the present study, we have overexpressed the major subtypes of cardiac beta-adrenergic receptors (beta(1)-AR and beta(2)-AR) in atrial myocytes by transient transfection. Whereas in native cells beta-adrenergic stimulation with isoproterenol failed to induce measurable GIRK current, robust currents were recorded from myocytes overexpressing either beta(1)-AR or beta(2)-AR. Whereas the beta(2)-AR-induced current showed the same sensitivity to pertussis toxin as the current evoked by the endogenous G(i/o)-coupled muscarinic M(2) receptor, isoproterenol-activated currents were insensitive to pertussis toxin treatment in beta(1)-AR-overexpressing myocytes. In contrast to a recent publication (Leaney, J. L., Milligan, G., and Tinker, A. (2000) J. Biol. Chem. 275, 921-929), sizable GIRK currents could also be activated by isoproterenol when the signaling pathway was reconstituted by transient transfection in two different standard cell lines (Chinese hamster ovary and HEK293). These results demonstrate that specificity of receptor-G protein signaling can be disrupted by overexpression of receptors. Moreover, the alpha subunit of heterotrimeric G proteins does not confer specificity to G beta gamma-mediated signaling.

Heterologous facilitation of G protein-activated K(+) channels by beta-adrenergic stimulation via cAMP-dependent protein kinase

To investigate possible effects of adrenergic stimulation on G protein-activated inwardly rectifying K(+) channels (GIRK), acetylcholine (ACh)-evoked K(+) current, I(KACh), was recorded from adult rat atrial cardiomyocytes using the whole cell patch clamp method and a fast perfusion system. The rise time of I(KACh ) was 0. 4 +/- 0.1 s. When isoproterenol (Iso) was applied simultaneously with ACh, an additional slow component (11.4 +/- 3.0 s) appeared, and the amplitude of the elicited I(KACh) was increased by 22.9 +/- 5.4%. Both the slow component of activation and the current increase caused by Iso were abolished by preincubation in 50 microM H89 (N-[2-((p -bromocinnamyl)amino)ethyl]-5-isoquinolinesulfonamide, a potent inhibitor of PKA). This heterologous facilitation of GIRK current by beta-adrenergic stimulation was further studied in Xenopus laevis oocytes coexpressing beta(2)-adrenergic receptors, m(2 )-receptors, and GIRK1/GIRK4 subunits. Both Iso and ACh elicited GIRK currents in these oocytes. Furthermore, Iso facilitated ACh currents in a way, similar to atrial cells. Cytosolic injection of 30-60 pmol cAMP, but not of Rp-cAMPS (a cAMP analogue that is inhibitory to PKA) mimicked the beta(2)-adrenergic effect. The possibility that the potentiation of GIRK currents was a result of the phosphorylation of the beta-adrenergic receptor (beta(2)AR) by PKA was excluded by using a mutant beta(2)AR in which the residues for PKA-mediated modulation were mutated. Overexpression of the alpha subunit of G proteins (Galpha(s)) led to an increase in basal as well as agonist-induced GIRK1/GIRK4 currents (inhibited by H89). At higher levels of expressed Galpha(s), GIRK currents were inhibited, presumably due to sequestration of the beta/gamma subunit dimer of G protein. GIRK1/GIRK5, GIRK1/GIRK2, and homomeric GIRK2 channels were also regulated by cAMP injections. Mutant GIRK1/GIRK4 channels in which the 40 COOH-terminal amino acids (which contain a strong PKA phosphorylation consensus site) were deleted were also modulated by cAMP injections. Hence, the structural determinant responsible is not located within this region. We conclude that, both in atrial myocytes and in Xenopus oocytes, beta-adrenergic stimulation potentiates the ACh-evoked GIRK channels via a pathway that involves PKA-catalyzed phosphorylation downstream from beta(2)AR.

Changes in function of cardiac receptors mediating the effects of the autonomic nervous system in the muscular dystrophy (MDX) mouse

Adrenergic and muscarinic receptor mediated effects on the force of contraction and heart rate were studied in the isolated left atria and right atria from dystrophin-deficient mdx mice and age matched C57BL/10ScSn (C57) mice, respectively. The pD(2) and pA(2) values of (-)-isoprenaline and CGP 20712A, respectively, were not different in left atria and right atria from mdx and C57 mice. (-)-Phenylephrine produced a small positive inotropic effect on mdx left atria that could be antagonized by prazosin, whereas in C57 left atria no positive inotropic response was seen. In contrast, the positive chronotropic effect of (-)-phenylephrine was reduced in right atria from mdx compared to C57 right atria (P<0.05). The potency and efficacy to carbachol in the presence of (-)-isoprenaline were higher in right atria from mdx compared to C57 mice (P<0.05), although in left atria only a greater efficacy was evident in mdx mice. In left atria, basal force of contraction and maximum Ca(2+)-induced increases in force of contraction were lower from mdx compared to C57 mice (P<0. 001 and P<0.05, respectively). In conclusion, marked changes were demonstrated in the function of alpha1-adrenoceptors and muscarinic receptors, but not in beta1-adrenoceptors in left and right atria from mdx mice.

Cardiac ionic currents and acute ischemia: from channels to arrhythmias

The aim of this review is to provide basic information on the electrophysiological changes during acute ischemia and reperfusion from the level of ion channels up to the level of multicellular preparations. After an introduction, section II provides a general description of the ion channels and electrogenic transporters present in the heart, more specifically in the plasma membrane, in intracellular organelles of the sarcoplasmic reticulum and mitochondria, and in the gap junctions. The description is restricted to activation and permeation characterisitics, while modulation is incorporated in section III. This section (ischemic syndromes) describes the biochemical (lipids, radicals, hormones, neurotransmitters, metabolites) and ion concentration changes, the mechanisms involved, and the effect on channels and cells. Section IV (electrical changes and arrhythmias) is subdivided in two parts, with first a description of the electrical changes at the cellular and multicellular level, followed by an analysis of arrhythmias during ischemia and reperfusion. The last short section suggests possible developments in the study of ischemia-related phenomena.

Antisense oligonucleotides against receptor kinase GRK2 disrupt target selectivity of beta-adrenergic receptors in atrial myocytes

K+ channels composed of GIRK subunits are predominantly expressed in the heart and various regions of the brain. They are activated by betagamma-subunits released from pertussis toxin-sensitive G-proteins coupled to different seven-helix receptors. In rat atrial myocytes, activation of K(ACh) channels is strictly limited to receptors coupled to pertussis toxin-sensitive G-proteins. Upon treatment of myocytes with antisense oligodesoxynucleotides against GRK2, a receptor kinase with Gbetagamma binding sites, in a fraction of cells, K(ACh) channels can be activated by beta-adrenergic receptors. Sensitivity to beta-agonist is insensitive to pertussis toxin treatment. These findings demonstrate a potential role of Gbetagamma binding proteins for target selectivity of G-protein-coupled receptors.

Activation of muscarinic K+ current by beta-adrenergic receptors in cultured atrial myocytes transfected with beta1 subunit of heterotrimeric G proteins

Muscarinic K+ channels (IK(ACh)) in native atrial myocytes are activated by betagamma subunits of pertussis toxin (Ptx)-sensitive heterotrimeric G proteins coupled to different receptors. betagamma subunits of Ptx-insensitive Gs, coupled to beta-adrenergic receptors, do not activate native IK(ACh). In atrial myocytes from adult rats transfected with rat brain beta1 subunit IK(ACh) can be activated by stimulation of beta-adrenergic receptors using isoprenaline. This effect is insensitive to Ptx. These findings demonstrate for the first time promiscuous (Ptx-insensitive) coupling of Gsbetagamma to GIRK channels in their native environment.

Acetylcholine-induced biphasic effect on the maximum upstroke velocity of action potential in mouse right atria: interaction with beta-adrenergic signaling cascade

Several lines of evidence suggest the molecular and functional entity of muscarinic M1 receptors in mammalian heart. We have reported that acetylcholine (ACh) reduces the maximum upstroke velocity of action potential (Vmax) through activation of muscarinic M1 receptors, which is followed by a muscarinic M2 receptor-mediated increase. The present study sought to determine whether activation of beta-adrenergic receptors modulates the muscarinic M1 and M2 receptor-mediated effects on Vmax in isolated mouse right atria. Intracellular recordings of spontaneous action potential were done using the conventional glass microelectrode technique. Isoproterenol (3 nM) completely antagonized ACh (5 microM)-induced reduction in Vmax. The antagonism was accompanied by a subsequent increase in Vmax. Propranolol (0.3 microM) abolished the effects of isoproterenol on ACh-induced changes in Vmax. Isoproterenol antagonized McN-A-343 (4-(m-chlorophenyl-carbamoyloxy)-2-butynyltrimethylammonium chloride) (300 microM, a muscarinic M1 receptor agonist)-induced reduction in Vmax. Oxotremorine (0.03 microM), a muscarinic M2 receptor agonist, did not affect Vmax by itself, but significantly increased it in the presence of 3 nM isoproterenol. The effects of isoproterenol were mimicked by cholera toxin (100 nM, 1 hr), a Gs-protein activator, and forskolin (10 nM), a direct activator of adenylyl cyclase. H-89 (N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulphonamide++ +, 1 microM), a selective protein kinase (PK)-A inhibitor, abolished the antagonism by isoproterenol of ACh-induced reduction in Vmax. The present results suggest that activation of the beta-adrenergic-Gs-adenylyl cyclase system antagonizes ACh-induced reduction (muscarinic M1-mediated) and potentiates the subsequent increase (muscarinic M2 receptor-mediated) in Vmax. The beta-adrenergic antagonism of ACh-induced reduction in Vmax may involve cross-talk between PK-A and PK-C signaling pathways.

Compartmentation of the type I regulatory subunit of cAMP-dependent protein kinase in cardiac ventricular muscle

The species-dependent compartmentation of type I cAMP-dependent protein kinase (PKA I) and its dissociated regulatory subunit (RI) was examined in the heart by biochemical and immunohistochemical means. PKA I and RI were resolved from type II cAMP-dependent protein kinase and its regulatory subunit by DEAE-Sephacel chromatography of the supernatant and Triton X-100 soluble particulate fractions of heart homogenates. The relative amounts of holoenzymes and subunits were determined by cAMP-binding, protein kinase, 8-N3-[32P]cAMP photoaffinity labeling, and Western blot assays. Rat, rabbit, and guinea pig hearts all contained PKA I to varying degrees, but only in the supernatant fractions. Significant amounts of dissociated RI were found in the supernatant fractions, and to a lesser extent the particulate fractions, of these species. In contrast, though no PKA I was detected in the supernatant or particulate fractions of pig and beef heart, half of the cAMP-binding activity in the particulate fraction was attributed to RI. The results suggest that RI may associate with membrane fractions when it is not associated with the PKA catalytic subunit. Immunohistochemical studies of tissue sections from pig, beef, and rat cardiac ventricle using antibodies directed against RI also revealed species-dependent localization of RI. Cardiac myocyte intercalated discs were stained in pig and beef sections with additional sarcolemmal staining in beef sections. Rat ventricle, which contained large amounts of supernatant PKA I, showed nuclear staining. The localization of RI to cardiac myocyte intercalated discs and sarcolemma in certain species suggests a role(s) for this subunit in mediating cAMP-regulated events in these regions.

Purinergic facilitation of ATP-sensitive potassium current in rat ventricular myocytes

1. The effects of different purinergic agonists on the cardiac adenosine 5'-triphosphate (ATP)-sensitive potassium current (IK(ATP)), appearing during dialysis of rat isolated ventricular myocytes with a low-ATP (100 microM) internal solution under whole-cell patch-clamp conditions, were examined in the presence of a P1 purinoceptor antagonist. 2. The extracellular application of ATP in the micromolar range induced, besides known inward currents through cationic and chloride channels, the facilitation of IK(ATP) once IK(ATP) had already been partially activated during the low-ATP dialysis. 3. Analogues of ATP, alpha, beta-methyleneadenosine 5'-triphosphate (alpha, beta meATP), 2-methylthioadenosine triphosphate (2MeSATP), adenosine 5'-O-3-thiotriphosphate (ATP gamma S) similarly facilitated IK(ATP). UTP and ADP were very weak agonists while AMP and adenosine had no detectable effect. 4. The half-maximal stimulating concentration (C50) of alpha, beta meATP, an analogue that did not elicite the interfering inward cationic current was 1.5 microM. Similar apparent C50 (1-2 microM) were observed for ATP and analogues tested with somewhat less maximal effect of ATP gamma S. 5. Suramin, a nonselective P2-purinoceptor antagonist, altered IK(ATP) at the relatively high concentration required to inhibit purinoceptors. Pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), a supposedly predominantly P2x-purinoceptor antagonist, at micromolar concentration inhibited the transient inward current but did not block the facilitation of IK(ATP). 6. Our results demonstrate that ATP and its analogues facilitate IK(ATP) in rat ventricular myocytes by stimulation of non-P1-, non-P2x-purinoceptors.

Calcium gradients during excitation-contraction coupling in cat atrial myocytes

1. Confocal microscopy in combination with the calcium-sensitive fluorescent probe fluo-3 was used to study spatial aspects of intracellular Ca2+ signals during excitation-contraction coupling in isolated atrial myocytes from cat heart. 2. Imaging of [Ca2+]i transients evoked by electrical stimulation revealed that Ca2+ release started at the periphery and subsequently spread towards the centre of the myocyte. 3. Blocking sarcoplasmic reticulum (SR) Ca2+ release with 50 microM ryanodine unmasked spatial inhomogeneities in the [Ca2+]i was higher in the periphery than in central regions of the myocyte. 4. Positive (or negative) staircase or postrest potentiation of the 'whole-cell' [Ca2+] transients were paralleled by characteristic changes in the spatial profile of the [Ca2+]i signal. With low SR Ca2+ load [Ca2+]i transients in the subsarcolemmal space were small and no Ca2+ release in the centre of the cell was observed. Loading of the SR increased subsarcolemmal [Ca2+]i transient amplitude and subsequently triggered further release in more central regions of the cell. 5. Spontaneous Ca2+ release from functional SR units, i.e. Ca2+ sparks, occurred at higher frequency in the subsarcolemmal space than in more central regions of the myocyte. 6. Visualization of the surface membrane using the membrane-selective dye Di-8-ANEPPS demonstrated that transverse tubules (t-tubules) were absent in atrial cells. 7. It is concluded that in atrial myocytes voltage-dependent Ca2+ entry triggers Ca2+ release from peripheral coupling SR that subsequently induces further Ca2+ release from stores in more central regions of the myocyte. Spreading of Ca2+ release from the cell periphery to the centre accounts for [Ca2+]i gradients underlying the whole-cell [Ca2+]i transient. The finding that cat atrial myocytes lack t-tubules demonstrates the functional importance of Ca2+ release from extended junctional (corbular) SR in these cells.