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

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 (V̇_max) was 418μmol l^–1 s^–1 in control conditions. ISO did not affect V̇_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 μmol l^–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 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 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.