Abstract 645: Sarcoplasmic Reticulum Ca2+ is Required for Catecholamine Stimulated Rate Increases, but not for ‘Intrinsic’ Automaticity in Sinoatrial Nodal Pacemaker Cells
Sarcoplasmic reticulum (SR) calcium is an important contributor to excitation-contraction coupling in ventricular myocytes, and is the major source of Ca2+ for contraction in myocardium. SR Ca2+ content therefore is an important determinant of cell contractility, but far less is known about the role of SR Ca2+ content in sinoatrial nodal (SAN) pacemaker cells. To investigate the role of SR calcium in pacemaking, we measured SR calcium content in isolated SAN cells from mice, and recorded spontaneous action potentials before and after ryanodine, using the perforated patch technique (35±1oC). SR Ca2+ content, normalized for cell membrane area, was significantly higher in SAN cells (1.65±0.16 C/F, n=16) than in ventricular myocytes (0.96±0.09 C/F, n=9, p=0.006), suggesting that SR Ca2+ plays an important role in SAN cell function. Ryanodine (0.1–10 μM) significantly reduced the automaticity rates of SAN cells (‘beats’/min) from 300±12 to 176±11 (n=7), (p<0.0001) in a concentration and time dependent manner. This rate slowing effect of ryanodine reached a common steady-state level at all concentrations tested. The persistence of spontaneous SAN activity after ryanodine, suggested SR calcium is not necessary for intrinsic pacemaking activity. Isoproterenol (ISO, 1 μM) increased SAN cells automaticity from 275±15 (n=13) to 338±17 (n=10) ‘beats’/min (P<0.01). However, ryanodine pretreatment prevented ISO increases in SAN cells automaticity: 176±11 basal (n=7) and 179±26 ‘beats’/min after ISO (n=5) (P=0.9). These data indicate that SR Ca2+ depletion prevents chronotropic responses to catecholamines in SAN cells and suggest that fight or flight responses in SAN cells and contracting ventricular myocytes both rely on a highly developed intracellular Ca2+ cycling apparatus.