Cyclic nucleotide phosphodiesterase in rat parotid gland

Ca(2+)/calmodulin-dependent cyclic nucleotide phosphodiesterase in cGMP metabolism in rabbit parotid acinar cells

Muscarinic cholinergic receptor activation provokes cGMP formation in parotid acinar cells. We investigated the involvement of Ca(2+)/calmodulin-dependent cyclic nucleotide phosphodiesterase (PDE1) in cGMP breakdown in rabbit parotid acinar cells. The muscarinic agonist carbachol stimulated cGMP formation in the cells. The carbachol-induced cGMP formation was enhanced in the presence of 8-methoxymethyl-3-isobutyl-1-methylxanthine (MM-IBMX), a PDE1 inhibitor. cGMPPDE activity in rabbit parotid acinar cells was reduced by about 25% in the absence of Ca(2+)/ calmodulin or in the presence of MM-IBMX. Ca(2+)/calmodulin-dependent cGMP-PDE in rabbit parotid acinar cells was purified using Calmodulin-Sepharose 4B and Mono Q ion-exchange column chromatography. Two dominant fractions with cGMP-PDE activity, referred to as the P-1 and P-2 fractions, were eluted from the Mono Q ion-exchange column. The Km values for cGMP of PDE in the P-1 and P-2 fractions were 0.82 microM and 0.40 microM, respectively, which were much lower than that for cAMP. The EC(50) for Ca(2+) and calmodulin of PDEs in the P-1 and P-2 fractions were 458 nM and 426 nM, respectively, and 32 nM and 137 nM, respectively. Protein bands that crossreacted with anti-PDE1A antibody were detected. These results suggest that Ca(2+)/calmodulin-dependent PDE, PDE1A, is involved in cGMP breakdown in rabbit parotid acinar cells.

Characterization of cyclic AMP phosphodiesterase isozymes in rat parotid gland

Cyclic AMP phosphodiesterase (PDE) isozymes were isolated and characterized from the soluble fraction of rat parotid gland. Four main peaks containing PDE activity were obtained by Q-Sepharose Fast Flow column chromatography. The four peaks were identified as PDEs I-IV by kinetic properties, molecular-weight analysis and their responses to effectors and inhibitors.

Ca2+/calmodulin-dependent protein phosphatase in bovine parotid gland: purification and characterization

Calmodulin-dependent protein phosphatase (CaM-PPase) was isolated from bovine parotid gland by sequential application of DEAE-52, Affi-gel blue and calmodulin-affinity chromatography followed by gel filtration and high performance liquid chromatography. The enzyme was activated in the simultaneous presence of Ni2+ or Mn2+ and Ca2+ plus calmodulin. Ca2+/calmodulin-dependent activation of CaM-PPase was antagonized by inhibitors of calmodulin action, such as W-7 and trifluoperazine. Tryptophan fluorescence was quenched in the presence of Ni2+. CaM-PPase was a heterodimer. The molecular weights of large subunits which bound calmodulin (CaM) were 68 kD and 58 kD - the 68 kD subunit was predominant. Polyclonal antibodies against bovine calcineurin cross-reacted with both types of larger subunits. Using polyclonal antibodies against bovine calcineurin or the monoclonal antibody against subunit B of bovine calcineurin, the smaller molecular weight subunit (19 kD) was found to be immunologically identical to subunit B of bovine calcineurin. In bovine parotid gland, CaM-PPase was found both in acinar and duct cells.

Stimulation-induced changes in cyclic nucleotide content by isolated rat parotid acini: effects of calcium

In the present study we investigated the relation between receptor activation and the acinar formation of cyclic nucleotides as a function of time, including effects of calcium. The experiments were performed using collagenase-isolated rat parotid acini and the cyclic nucleotide content was determined using a specific cAMP binding protein and cGMP antibody. Under physiological conditions, activation of beta-adrenoceptors increased cAMP content 27 fold within 60 sec. This response was not dependent upon the presence of extracellular calcium nor could it be mimicked by the calcium ionophore A23187. The adrenaline-induced activation of the alpha-adrenergic receptors lead to a 19% increase in cGMP content within 15 sec. This response was abolished in the absence of extracellular calcium. Pure activation of the alpha-1-adrenoceptors by phenylephrine resulted in a 62% increase in cGMP content within 15 sec. Finally, carbachol-induced activation of the cholinoceptors lead to a 50% transient increase in cGMP content within 7 sec. In the absence of calcium (EGTA present), the basal cGMP level was increased and in addition, the carbachol-induced cGMP formation was slower in onset and amounted to only 23% within 60 sec. The carbachol-induced cGMP formation could be mimicked by the calcium ionophore A23187 or by addition of calcium to acini preincubated in a calcium free buffer containing A23187. In conclusion, stimulation-induced formation of cyclic nucleotides under various conditions proved to be very rapid, well regulated, and sensitive processes.

Mechanism of action of extracellular calcium on isoprenaline-evoked amylase secretion from isolated rat parotid glands

The effects of extracellular Ca2+ deprivation on amylase secretion, 45Ca efflux and cyclic adenosine 3',5'-monophosphate (cyclic AMP) metabolism were investigated in incubated parotid glands of young rats. Reducing the extracellular Ca2+ concentration from 2.5 X 10(-3) M to 10(-6) M had no effect on amylase secretion but did increase the rate of 45Ca efflux from unstimulated glands. Isoprenaline (10(-5) M) increased amylase secretion and the rate of 45Ca efflux in media containing 2.5 X 10(-3) M-Ca2+, and also increased the parotid cyclic AMP content. When glands were pre-incubated for either 10, 20, or 30 min in media containing either 10(-3) or 10(-6) M-Ca2+ there was no effect on the peak rate of amylase secretion stimulated by isoprenaline but the response took longer to develop. The effects of the same experimental manoeuvre on the net change in 45Ca efflux stimulated by isoprenaline were consistent with the idea that the beta-adrenergic agonist mobilizes Ca2+ from a number of intracellular pools. When glands were pre-incubated for 100 min in Ca2+-depleted buffers and then exposed to isoprenaline a triphasic effect on amylase secretion was observed. The secretory response was depressed when the extracellular Ca2+ concentration was lowered from 2.5 X 10(-3) M to 10(-3) M; remained unaffected when the extracellular Ca2+ was further reduced to 10(-5) M; and was virtually abolished when the extracellular Ca2+ concentration was decreased to 10(-6) M. When glands were pre-incubated for 100 min in media containing Ca2+ at concentrations above 10(-5) M and then stimulated with isoprenaline parallel changes in cyclic AMP content and amylase secretion were observed. At lower Ca2+ concentrations there was no further reduction in cyclic AMP content. The lipophilic cyclic AMP analogue N6,O2-dibutyryl adenosine 3',5'-monophosphate (dibutyryl cyclic AMP) mimicked the effects of isoprenaline on amylase secretion and 45Ca efflux. However, the amylase secretory response took longer to develop and the 45Ca efflux response was of shorter duration. These responses were reduced after pre-incubation of glands for 100 min in a medium containing 10(-6) M-Ca2+. Ca2+ at concentrations up to 10(-7) M enhanced parotid adenylate cyclase activity but higher concentrations were inhibitory. The same pattern of Ca2+ sensitivity was seen for basal and also isoprenaline and F--stimulated activities. The results show that isoprenaline can mobilize Ca2+ from a number of intracellular pools which may exchange at different rates with the extracellular medium. This Ca2+ may be required at a minimum of two sites in the amylase secretory pathway.(ABSTRACT TRUNCATED AT 400 WORDS)