Capacitative calcium entry in parotid acinar cells.
The intracellular Ca2+ indicator, fura-2, was used to monitor changes in cytosolic [Ca2+] in parotid acinar cells. When parotid cells were incubated in a medium containing low [Ca2+], and [Ca2+] was restored to the physiological range, there was a small increase in cytosolic [Ca2+]. If, however, the cells were first activated by a muscarinic agonist, and receptor activation was terminated before the addition of Ca2+ by the addition of a pharmacological excess of the muscarinic-receptor antagonist atropine, the initial increase in cytosolic [Ca2+] was faster and transiently larger than in the control cells which had not been previously stimulated. This suggested that a stimulation of Ca2+ entry occurred owing to the prior emptying of the agonist-regulated intracellular Ca2+ pool. This extra Ca2+ influx seen in pool-depleted cells persisted even when the interval between the addition of atropine and Ca2+ was increased from 1 to 20 min. Also, when the pool was allowed to refill by adding atropine in the presence of extracellular Ca2+, and Ca2+ was then sequentially removed and restored, the rise in cytosolic [Ca2+] after the addition of extracellular Ca2+ was not rapid, and resembled the increase seen in unstimulated cells. These results indicate that, when the agonist-sensitive Ca2+ pool is emptied by an agonist, Ca2+ influx across the plasma membrane is increased. This influx of Ca2+ occurs independently of the concentrations of inositol phosphates and probably of any second messengers linked directly to receptor activation. It appears rather to be a consequence of the empty state of the Ca2+ pool. Further, we suggest that, whenever the agonist-sensitive Ca2+ pool is emptied by agonist activation, the plasma-membrane permeability to Ca2+ will be increased, and this may account, at least in part, for the phenomenon of receptor-activated Ca2+ entry.
Takemura
H
H
Calcium Regulation Section, National Institute of Environmental Health Sciences, Research Triangle Park, NC 27709.
Putney
J W
JW
D000818:Animals; D001285:Atropine / Q000494:pharmacology; D001572:Benzofurans; D002118:Calcium / Q000378:metabolism*; D002478:Cells, Cultured; D003600:Cytosol / Q000378:metabolism; D005456:Fluorescent Dyes; D016257:Fura-2; D016210:Methacholine Chloride; D008688:Methacholine Compounds / Q000494:pharmacology; D010306:Parotid Gland / Q000166:cytology* / Q000187:drug effects / Q000378:metabolism; D051381:Rats; D011976:Receptors, Muscarinic / Q000378:metabolism
D001572:Benzofurans; D005456:Fluorescent Dyes; D008688:Methacholine Compounds; D011976:Receptors, Muscarinic; D016210:Methacholine Chloride; D001285:Atropine; D002118:Calcium; D016257:Fura-2