Subcellular Propagation of Cardiomyocyte β-Adrenergic Activation of Calcium Uptake Involves Internal β-Receptors and AKAP7

Abstract

β-adrenergic receptor (β-AR) signalling in cardiac myocytes is central to cardiac function, but spatiotemporal activation within myocytes is unresolved. In rabbit ventricular myocytes β-AR agonists or high extracellular [Ca2+] were applied locally at one end, to measure β-AR signal propagation as Ca2+− transient (CaT) amplitude and sarcoplasmic reticulum (SR) Ca2+ uptake. High local [Ca]o, increased CaT amplitude under the pipette faster than did ISO, but was also more spatially restricted. Local isoproterenol (ISO) or norepinephrine (NE) increased CaT amplitude and SR Ca2+ uptake, that spread along the myocyte to the unexposed end. Thus, local [Ca]i decline kinetics reflect spatio-temporal progression of β-AR end-effects in myocytes. To test whether intracellular β-ARs contribute to this response, we used β-AR-blockers that are membrane permeant (propranolol) or not (sotalol). Propranolol completely blocked NE-dependent CaT effects. However, blocking surface β-ARs only (sotalol) suppressed only ∼50% of the NE-induced increase in CaT peak and rate of [Ca2+]i decline, but these changes spread more gradually than NE alone. We also tested whether A-Kinase Anchoring Protein 7γ (AKAP7γ; that interacts with phospholamban) is mobile, such that it might contribute to intracellular spatial propagation of β-AR signalling. We found AKAP7γ to be highly mobile using fluorescence recovery after photobleach of GFP tagged AKAP7γ, and that PKA activation accelerated AKAP7γ-GFP wash-out upon myocyte saponin-permeabilization, suggesting increased AKAP7γ mobility. We conclude that local β-AR activation can activate SR Ca2+ uptake at remote myocyte sites, and that intracellular β-AR and AKAP7γ mobility may play a role in this spread of activation.

Hypoxia Sensing of β-Adrenergic Receptor Is Regulated by Endosomal PI3Kγ

“…It is known that endosomal βARs can be activated by agonist 23,24 like intracellular βAR activation mediates calcium transients in cardiomyocytes. 25 This suggests that endosomal dephosphorylated βARs can be engaged by agonists resulting in measurable cellular events. Since hypoxia results in increased endosomal βAR phosphorylation, hypoxia could selectively inhibit endosomal receptor signaling.…”

Mobile Anchors

“…The least one expects from an anchor is to stay put (freely adapted from Salvador Dali's opinion about Alexander Calder's sculptures). The article by Shannon and co-workers in this issue 1 contradicts such expectation, thus contributing an original and crucial tile to the puzzle of “nanodomain” signaling by β-adrenergic receptors (β-AR).…”

“…Shannon and coworkers 1 address this issue by combining a localized perfusion strategy and FRET-based technology to β1-AR signaling in native rabbit ventricular myocytes. They show that the PLN-targeted intracellular β1AR-PKA-AKAP7γ complex they recently identified 9 is endowed with considerable mobility, exceeding that of PLN itself.…”

“…Interestingly, under sotalol, NE effect linearly increased throughout the 90 s recording; this was in sharp contrast with the rapidly saturating (within 50 s) response observed in the absence of sotalol. 1 Thus, distinct signaling modalities, one recruiting sarcolemmal and intracellular β-ARs and the other based on intracellular ones only, modulate PLN with remarkably different kinetics. In terms of mechanism, short-range modulation of the β-AR signal within the AKAP nanodomain ( Figure 1 ) might possibly account for signaling kinetics unlike those expected from a “bulk” signal activation—inactivation sequence.…”

Beta-blockers in cardiac arrhythmias–Clinical pharmacologist’s point of view

“…Research indicates the presence of intracellular β1-adrenoceptors in myocytes, which, by generating a local signal, may be involved in accelerating Ca2 + uptake ( Shannon et al, 2022 ). Beta-blockers differ in their permeability across cell membranes.…”