RNA Isolation and Northern Blot Analysis-Total RNA was extracted by previously published methods (31) from cells on 100-mm dishes. Total RNA (20 g) was denatured, separated, and transferred to a nylon solid support. Blots were hybridized with [␣-32 P]dCTP-labeled probes of an 1.8-kb BamHI-NotI fragment of the rat PTH/PTHrP receptor cDNA (32), a 0.89-kb AvaII fragment of human heart Cx43 cDNA (29), a 0.52-kb EcoRI fragment of rat osteocalcin cDNA (33), an 1.8-kb PstI-HindIII fragment of the chicken type I collagen cDNA (34), or an 1.4-kb PstI fragment of rat GAPDH cDNA (35). Hybridizations and washes were performed according to the manufacturer's instructions (Schleicher & Schuell), followed by exposures for various time periods to Kodak X-Omat AR film with a Cronex intensifying screen. The abundance of specific mRNAs was quantified using a Bio-Rad Imaging Densitometer. The **data** were normalized to GAPDH mRNA abundance.