Does beta-adrenoceptor activation stimulate Ca2+ mobilization and inositol trisphosphate formation in parotid acinar cells?
The effects of the beta-adrenoceptor agonist, isoprenaline, on Ca2+ mobilization and inositol phosphate formation in parotid acinar cells were examined. Isoprenaline (2 microM) failed to increase cytosolic [Ca2+] in acinar cells, as measured by Fura-2 fluorescence, even in the presence of a phosphodiesterase inhibitor. Likewise, neither the 8-bromo nor the dibutyryl derivatives of cAMP (both at 2 mM concentration) increased [Ca2+]i. However, in confirmation of results previously published, a higher concentration of isoprenaline (200 microM) increased cytosolic [Ca2+]i of rat parotid acinar cells, from 104 +/- 4 nM to 151 +/- 18 nM. The increase in [Ca2+]i in response to isoprenaline, while transient in the absence of extracellular Ca2+, was sustained in Ca2(+)-containing medium. This isoprenaline-stimulated Ca2+ signal was more potently antagonized by phentolamine than by propranolol, suggesting that the higher concentration of isoprenaline activated alpha-adrenoceptors. Furthermore, the Ca2+ signal generated in response to the alpha-adrenoceptor agonist, phenylephrine, also was blocked by the same concentrations of propranolol necessary to block the effects of isoprenaline, suggesting that propranolol may block alpha-adrenoceptors under certain experimental conditions. The high concentration of (-)isoprenaline (200 microM) also increased inositol (1,4,5) trisphosphate and inositol (1,3,4) trisphosphate formation 45% within 30 s. Analogous to the increase in intracellular Ca2+, the formation of inositol phosphates stimulated by isoprenaline was more potently antagonized by the alpha-adrenoceptor antagonist, phentolamine, than by the beta-adrenoceptor antagonist, propranolol, again suggesting that isoprenaline interacts with alpha-adrenoceptors on parotid cells. Thus, the effects of isoprenaline on [Ca2+]i do not appear to be mediated by cAMP.(ABSTRACT TRUNCATED AT 250 WORDS)
Hughes
A R
AR
Calcium Regulation Section, National Institute of Environmental Health Sciences, National Institute of Health, North Carolina.
Takemura
H
H
Putney
J W
JW
D015124:8-Bromo Cyclic Adenosine Monophosphate / Q000494:pharmacology; D000818:Animals; D001572:Benzofurans; D003994:Bucladesine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D003600:Cytosol / Q000378:metabolism; D005456:Fluorescent Dyes; D016257:Fura-2; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism*; D007545:Isoproterenol / Q000494:pharmacology; D016210:Methacholine Chloride; D008688:Methacholine Compounds / Q000494:pharmacology; D010306:Parotid Gland / Q000187:drug effects / Q000378:metabolism*; D010646:Phentolamine / Q000494:pharmacology; D011433:Propranolol / Q000494:pharmacology; D051381:Rats; D011943:Receptors, Adrenergic, beta / Q000187:drug effects / Q000502:physiology*; D013050:Spectrometry, Fluorescence
D001572:Benzofurans; D005456:Fluorescent Dyes; D008688:Methacholine Compounds; D011943:Receptors, Adrenergic, beta; D016210:Methacholine Chloride; D015124:8-Bromo Cyclic Adenosine Monophosphate; D003994:Bucladesine; D015544:Inositol 1,4,5-Trisphosphate; D011433:Propranolol; D007545:Isoproterenol; D002118:Calcium; D016257:Fura-2; D010646:Phentolamine