Cardiac fibroblasts form one of the largest cell populations, in terms of cell numbers, in the heart. They contribute to structural, biochemical, mechanical and electrical properties of the myocardium. Nonetheless, they are often disregarded by in vivo and in vitro studies into cardiac function. This review summarizes our understanding of fibroblast origin and identity, their structural organization and role in myocardial architecture, as well as functional aspects related to cell signalling and electro-mechanical function in the heart.

Cardiomyocytes form a conducting network that is assumed to be electrically isolated from nonmyocytes in vivo. In cell culture, however, cardiac fibroblasts can contribute to the spread of excitation via functional gap junctions with cardiomyocytes. To assess the ability of fibroblasts to form gap junctions in vivo, we combine in situ detection of connexins in rabbit sinoatrial node (a tissue that is particularly rich in fibroblasts) with identification of myocytes and fibroblasts using immunohistochemical labeling and confocal microscopy. We distinguish two spatially distinct fibroblast populations expressing different connexins: fibroblasts surrounded by other fibroblasts preferentially express connexin40, whereas fibroblasts that are intermingled with myocytes largely express connexin45. Functionality of homogeneous and heterogeneous cell coupling was investigated by dye transfer in sinoatrial node tissue explants. These studies reveal spread of Lucifer yellow, predominantly along extended threads of interconnected fibroblasts (probably via connexin40), and occasionally between neighboring fibroblasts and myocytes (probably via connexin45). Our findings show that cardiac fibroblasts form a coupled network of cells, which may be functionally linked to myocytes in rabbit SAN.

Significance: Heart pumping is triggered and coordinated by action potentials (APs) originating in and spreading among electrically excitable heart muscle cells (myocytes) via electrotonic coupling. Cardiac nonmyocytes are thought not to participate in AP conduction in situ, although heterocellular electrotonic coupling is common in cell culture. We used optogenetic tools involving cell-specific expression of a voltage-reporting fluorescent protein to monitor electrical activity in myocytes or nonmyocytes of mouse hearts. We confirm the suitability of this technique for measuring cell type-specific voltage signals and show that, when expressed in nonmyocytes, myocyte AP-like signals can be recorded in cryoinjured scar border tissue. This direct evidence of heterocellular electrotonic coupling in the whole heart necessitates a review of current concepts on cardiac electrical connectivity.

We investigate acute effects of axial stretch, applied by carbon fibers (CFs), on diastolic Ca ²⁺ spark rate in rat isolated cardiomyocytes. CFs were attached either to both cell ends (to maximize the stretched region), or to the center and one end of the cell (to compare responses in stretched and nonstretched half-cells). Sarcomere length was increased by 8.01±0.94% in the stretched cell fraction, and time series of XY confocal images were recorded to monitor diastolic Ca ²⁺ spark frequency and dynamics. Whole-cell stretch causes an acute increase of Ca ²⁺ spark rate (to 130.7±6.4%) within 5 seconds, followed by a return to near background levels (to 104.4±5.1%) within 1 minute of sustained distension. Spark rate increased only in the stretched cell region, without significant differences in spark amplitude, time to peak, and decay time constants of sparks in stretched and nonstretched areas. Block of stretch-activated ion channels (2 μmol/L GsMTx-4), perfusion with Na ⁺ /Ca ²⁺ -free solution, and block of nitric oxide synthesis (1 mmol/L L-NAME) all had no effect on the stretch-induced acute increase in Ca ²⁺ spark rate. Conversely, interference with cytoskeletal integrity (2 hours of 10 μmol/L colchicine) abolished the response. Subsequent electron microscopic tomography confirmed the close approximation of microtubules with the T-tubular-sarcoplasmic reticulum complex (to within ≈10 ⁻⁸ m). In conclusion, axial stretch of rat cardiomyocytes acutely and transiently increases sarcoplasmic reticulum Ca ²⁺ spark rate via a mechanism that is independent of sarcolemmal stretch-activated ion channels, nitric oxide synthesis, or availability of extracellular calcium but that requires cytoskeletal integrity. The potential of microtubule-mediated modulation of ryanodine receptor function warrants further investigation.