The Role of Cyclic Nucleotide Pathways and Calmodulin in Ciliary Stimulation

Abstract

Cilia are tiny hairlike protrusions enveloped by a membrane contiguous with the cell membrane, which beat in a cooperative pseudo-periodic, spatial and temporal pattern called the metachronal wave. They are thin (0.25–0.3 µm), relatively long (6–50 µm) and densely packed on the cell surface (100–200 cilia per cell). In the mucociliary system, their primary function is transport of a mucus layer over the cell together with various objects that may be trapped in this layer. Highly cooperative beating of cilia at high frequencies enables the mucociliary system to carry relatively large objects, at remarkable velocities. Furthermore, high frequency of ciliary beating results in increased energy expenditure. Therefore, under normal conditions, cilia beat with either low frequency or may even be at rest. However, they can dramatically change their activity in response to a variety of receptor-mediated stimuli. For example, ciliary cells possess purinergic P1 and P2 [7], [12], [32], [33], cholinergic [1], [6], [24], adrenergic [19], [31], [34] receptors.