Effect of beta-adrenergic stimulation on the relationship between membrane potential, intracellular [Ca2+] and sarcoplasmic reticulum Ca2+ uptake in rainbow trout atrial myocytes

摘要

Long depolarizations cause a steady tonic contraction and induce sarcoplasmic reticulum (SR) Ca2+-uptake in trout atrial myocytes. Simultaneous measurements of cytosolic [Ca2+] ([Ca2+](i)) and whole membrane current showed an elevated [Ca2+](i) throughout the depolarization. Rapid caffeine (Caf) applications at -80 mV before and after a long depolarization were used to determine SR Ca2+ loading and its dependency on membrane potential and [Ca2+](i) during depolarization. Following a 10 s depolarization, the maximal SR Ca2+ load was 597 mumol 1(-1) and loading was half-maximal at -12 mV. The beta-adrenergic agonist isoproterenol (ISO) did not affect the maximal SR Ca2+ loading but shifted the potential for half-maximal loading by -26 mV. Following a 3 s depolarization, the maximal SR Ca2+ uptake rate (V-max) was 418 mumol 1(-1) s(-1) in control conditions. ISO did not affect V-max, but significantly lowered the average free Ca2+ transient during the depolarization and shifted the K-0.5 for the relationship between SR Ca2+ uptake and [Ca2+](i) from 1.27 in control to 0.8 mumol 1(-1) with ISO. Following repetitive 200 ms depolarizations, ISO increased the (L)-type Ca2+ current (I-Ca) amplitude by 91+/-29% and the peak Ca2+ transient by 41+/-10%, and decreased the half life of the Ca2+ transient from 151+/-12 to 111+/-6 ms. Using the relationship between [Ca2+](i) and SR Ca2+ uptake to calculate the total SR Ca2+ uptake during a Ca2+ transient elicited by a 200 ms depolarization, a significant increase in the SR Ca2+ uptake from 37+/- 6 mumol 1(-1) in control to 68+/-4 mumol 1(-1) with ISO was seen. When normalized to the total Ca2+ transport the contribution of the SR was not significantly different in the absence (35+/-6%) or presence of ISO (41+/-4%). Exposure of cells to ISO and low extracellular [Ca2+] increased I-Ca by 67+/-40% (N=5) but significantly reduced SR Ca2+ uptake at membrane potentials above -30 mV. Together, these results suggest that (i) ISO has a stimulatory effect on the SR Ca2+ pump that may contribute to the faster decay of the Ca2+ transient, and (ii) the relative contribution of the SR to the Ca2+ removal during relaxation is not altered by ISO in trout atrial myocytes.