Beta-Adrenergic Signaling in Isolated Cardiomyocytes Propagates Spatially Over Time

Abstract

Increased β-adrenergic tone is implicated in cardiac arrhythmias We have tested the hypothesis that the response to isoproterenol (ISO) spreads spatially in a time-dependent manner. Isolated rabbit cardiomyocytes were exposed to ISO locally via pipette placed at its downstream end. Calcium transients were measured along a confocal scan line extending from underneath the ISO pipette to the other end. The peak Ca transients underneath the pipette increased 88.5±11.3% and transient decline was accelerated. Peak transients got gradually smaller with a slower decline with distance from the local ISO exposure. Transients rose only 42.4±5.7% at 36.4 μM distal to the pipette. Time from ISO application to 50% peak Ca transients increased from 22.4±1.7 under the pipette to 29.2±2.3 s. 20 μM away (t-test, p<0.05, n = 7). To verify that delayed activation was a property of the adrenergic response we used 4 mM Ca instead of ISO in the pipette to increase the Ca transient magnitude without activating second messengers. Ca transients increased by 60.7±14.7% of pre-ISO levels underneath the pipette. As was the case with local perfusion of ISO, the Ca transient magnitude rose only 30.1±8.1% at 18.4±2.6 μM distal to the pipette. However, the time to 50% increase in peak calcium transients remained the same with increasing distance away from the site of local Ca perfusion (12.5±1.0 Vs. 13.3±0.7 s. at 20 μM distal from the pipette, t-test, n=6). We conclude that the magnitude of the response to local increases in Ca signaling propagates decrementally away from a focal site of stimulation. Further, the data suggest that there is an additional delay in the response to adrenergic stimulation due to the spatial spread of second messengers that must carry the response distally in the cell.