To investigate how isoproterenol, a P-adrenergic agonist, regulates airway ciliary beat frequency (CBF), we simultaneously quantified changes in rabbit airway CBF and intracellular calcium concentration ([Ca2+](i)) in response to isoproterenol (100 mu mol/L) by using high-speed (240 frames per second) and fluorescence microscopy. At approximately 30 degrees C, isoproterenol induced a change in CBF that could be separated into two parts. First, isoproterenol induced a moderate increase in the basal CBF that was calcium-independent. This response was unaffected by buffering the [Ca2+](i) with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrapotassium salt (BAPTA). Second, isoproterenol induced a transient increase in CBF that was superimposed on the increased basal CBF and correlated with a simultaneous transient increase in [Ca2+](i). This transient increase in CBF was abolished by BAPTA. We conclude that isoproterenol initially increases CBF through a calcium-independent mechanism, probably via protein kinase A, and subsequently through a calcium-dependent mechanism mediated by an increase in [Ca2+](i).