Effect of beta-adrenergic stimulation on the relationship between membrane potential, intracellular [Ca2+] and sarcoplasmic reticulum Ca2+ uptake in rainbow trout atrial myocytes.
Long depolarizations cause a steady tonic contraction and induce sarcoplasmic reticulum (SR) Ca(2+)-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 micromol l(-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 (Vmax) was 418 micromol l(-1) s(-1) in control conditions. ISO did not affect Vmax, 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 micromol 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 micromol l(-1) in control to 68+/-4 micromol l(-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 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.
Llach
Anna
A
Unitat de Fisiologia Animal, Departamento de Biologia Celular, Fisiologia i Immunología, Facultat de Ciencies, Universitat Autònoma de Barcelona, 08193, Cerdanyola, Barcelona, España.
Huang
Jingbo
J
Sederat
Franklin
F
Tort
Lluis
L
Tibbits
Glen
G
Hove-Madsen
Leif
L
D000318:Adrenergic beta-Agonists / Q000494:pharmacology*; D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects; D002110:Caffeine / Q000378:metabolism / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism*; D004594:Electrophysiology; D007545:Isoproterenol / Q000494:pharmacology*; D008564:Membrane Potentials / Q000187:drug effects; D032383:Myocytes, Cardiac / Q000187:drug effects* / Q000378:metabolism; D017686:Oncorhynchus mykiss / Q000378:metabolism / Q000502:physiology*; D012519:Sarcoplasmic Reticulum / Q000187:drug effects* / Q000378:metabolism
D000318:Adrenergic beta-Agonists; D002110:Caffeine; D007545:Isoproterenol; D002118:Calcium