Effects of thapsigargin on aequorin-injected and skinned preparations of ferret ventricular muscles.
OBJECTIVE
We investigated the effects of thapsigargin (TG) (0.1-1 microM) on the relation between intracellular Ca2+ concentration and tension in ferret papillary muscles using aequorin-injected and skinned preparations.


METHODS
Aequorin was injected into the superficial cells of ferret papillary muscles; the Ca2+ signals of aequorin and tension in twitch and those with the application of 15 mM caffeine were simultaneously measured. The alteration of Ca2+ sensitivity of the contractile elements was examined by measuring the pCa-tension relation in Triton-X-treated skinned preparations.


RESULTS
TG decreased the peak of the Ca2+ signal accompanied by a prolonged decay time. However, the tension was scarcely altered even at 1 microM TG. TG inhibited the caffeine-induced Ca2+ signal. Prolongation of decay of the Ca2+ signal by TG in twitch was further enhanced by isoprenaline (10 nM). The pCa-tension relation of the skinned preparation was slightly but significantly shifted to the right by TG.


CONCLUSIONS
The apparent dissociation of the effects of TG on the Ca2+ signal and tension in intact preparations is not a result of alteration of the Ca2+ sensitivity of the myofilaments. The effects of TG in multicellular preparations are probably limited to the outer layer of the preparation. The slower time course of the Ca2+ signal induced by TG is due to the inhibition of Ca2+ uptake by sarcoplasmic reticulum, which is more significantly observed when the intracellular Ca2+ transient is increased by isoprenaline.
Tanaka
T
T
Department of Physiology, Jikei University School of Medicine, Tokyo, Japan.
Komukai
K
K
Kawai
M
M
Tanaka
E
E
Kurihara
S
S
D000331:Aequorin / Q000494:pharmacology*; D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors*; D002316:Cardiotonic Agents / Q000494:pharmacology; D005289:Ferrets; D066298:In Vitro Techniques; D007545:Isoproterenol / Q000494:pharmacology; D010210:Papillary Muscles / Q000187:drug effects* / Q000378:metabolism; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism; D013729:Terpenes / Q000494:pharmacology*; D019284:Thapsigargin
D002316:Cardiotonic Agents; D013729:Terpenes; D002110:Caffeine; D000331:Aequorin; D019284:Thapsigargin; D000252:Calcium-Transporting ATPases; D007545:Isoproterenol; D002118:Calcium