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Phosphodiesterase (PDE) 3 has been characterized in isolated rat submandibular acini. PDE3 activity was detected in homogenates of isolated rat submandibular acini; little or no PDE3 activity was found in ducts. About 62% of PDE3 activity in the acini was recovered in the supernatant fractions; 38% in particulate fractions. In the acini, but not ducts, PDE3A mRNA was detected by reverse transcriptase-polymerase chain reaction (RT-PCR). The PDE3-specific inhibitor, cilostamide, increased the ratio of apomucin mRNA/18s rRNA, as quantified by real-time RT-PCR. Our results indicate that PDE3A may be important in regulating cAMP pools that control acini functions.

A recent preliminary (unpublished) study showed that phosphodiesterase (PDE) 3A and 3B are expressed in rat submandibular glands. Here, PDE3 activity was detected in homogenates of rat submandibular gland acinar epithelial (SMIE) cells, but not rat A5 (epithelial duct) cells. Most of the PDE3 activity in SMIE cells was recovered in the particulate fraction. Only PDE3B mRNA was detected by reverse transcription-polymerase chain reaction in RNA from SMIE cells. The nucleotide sequence of the fragment was identical to the sequence of rat PDE3B. The PDE3 specific inhibitor, OPC3689 (10 and 50 microM), inhibited the growth of SMIE cells (19 and 63%), but not A5 cells. As the submandibular gland contains many types of cells, these results indicate that PDE3B may regulate a cAMP pool that is important in submandibular gland acinar epithelial cell function.

We have evaluated effects of a phosphodiesterase (PDE) 4 inhibitor on retinoic acid-increased alkaline phosphatase activity in the mouse fibroblastic C3H10T1/2 clone 8 (10T1/2) cell line. 10T1/2 cells were cultured in minimum essential medium (MEM) and 10% fetal bovine serum with or without 1 microM retinoic acid and/or the PDE 4 inhibitor, rolipram, and harvested at specific intervals before measurement of alkaline phosphatase activity, cAMP production in response to parathyroid hormone, osteocalcin synthesis and expression, and phosphodiesterase activity. Retinoic acid-increased alkaline phosphatase activity, and slightly enhanced cAMP production in response to parathyroid hormone. However, it did not affect osteocalcin synthesis and expression. In the presence of retinoic acid, PDE 4 activity was not changed. A PDE 4 inhibitor, rolipram, and cAMP analog, 8-bromo-cAMP dramatically increased retinoic acid's ability to induce alkaline phosphatase activity. This is the first report that PDE 4 may be involved in regulation of retinoic acid-increased alkaline phosphatase activity.