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

Abstract

SUMMARY 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 μmol l–1 and loading was half-maximal at –12 mV. Theβ-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 (V̇max) was 418μmol l–1 s–1 in control conditions. ISO did not affect V̇max, but significantly lowered the average free Ca2+ transient during the depolarization and shifted the K0.5 for the relationship between SR Ca2+ uptake and [Ca2+]i from 1.27 in control to 0.8 μmol l–1 with ISO. Following repetitive 200 ms depolarizations, ISO increased the l-type Ca2+current (ICa) 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 μmol l–1 in control to 68±4 μmol l–1with 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 ICa 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.