Screening of Antihypertensive Drugs for Osteoclastic Acid Secretion and Bone Resorption by Acridine Orange

Both osteoporosis and high blood pressure are major diseases in the recent aging society and may share the same background genetically and environmentally. Although several antihypertensive drugs, thiazides, beta (β)-blockers and angiotensin converting enzyme (ACE) inhibitors, are reported to decrease the risk of bone fractures, calcium channel blockers (CCBs), which are widely used as first line drugs, have not shown significant effects. As treatments to prevent aging- related diseases simultaneously are desirable, a simple approach to identify drugs effective for both diseases is essential. In this study, we screened various antihypertensive drugs with a focus on osteoclast function of acid secretion, bone resorption and reactive oxygen species (ROS) activity. Extracellular acidification in the resorption lacunae of osteoclasts was identified by acridine orange staining under a fluorescence microscope. Low pH of activated osteoclasts was identified as red to orange waves, while neutral pH of resting osteoclasts was identified as yellow to green waves. These phenomena were well correlated with the findings of excavated pit areas formed on the synthetic bone surface by active or resting stage osteoclasts, respectively. It was shown that benidipine, cilnidipine, olmesartan and calvedilol strongly attenuated acid secretion of osteoclasts, azelnidipine moderately, and hydrochlorothiazide mildly reduced the secretion, while amlodipine and telmisartan did not. Overall, screening drugs by osteoclast acidification by fluoresence microscopy might be a useful approach for evaluating the action of drugs on bone metabolism.