PKA induces Ca²⁺ release and enhances ciliary beat frequency in a Ca²⁺-dependent and -independent manner

The intent of this work was to evaluate the role of cAMP in regulation of ciliary activity in frog mucociliary epithelium and to examine the possibility of cross talk between the cAMP- and Ca²⁺dependent pathways in that regulation. Forskolin and dibutyryl cAMP induced strong transient intracellular Ca²⁺ concentration ([Ca²⁺](i)) elevation and strong ciliary beat frequency enhancement with prolonged stabilization at an elevated plateau. The response was not affected by reduction of extracellular Ca²⁺ concentration. The elevation in [Ca²⁺](i) was canceled by pretreatment with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-AM, thapsigargin, and a phospholipase C inhibitor, U-73122. Under those experimental conditions, forskolin raised the beat frequency to a moderately elevated plateau, whereas the initial strong rise in frequency was completely abolished. All effects were canceled by H-89, a selective protein kinase A (PKA) inhibitor. The results suggest a dual role for PKA in ciliary regulation. PKA releases Ca²⁺ from intracellular stores, strongly activating ciliary beating, and, concurrently, produces moderate prolonged enhancement of the beat frequency by a Ca²⁺independent mechanism.