Beat Frequency and Ca2

Beat Frequency and Ca²⁺ Oscillations
The addition of extracellular ATP to ciliated epithelial cells results in an initial large increase in [Ca²⁺]_i (RED traces and images A though D, right). After about 30 seconds, the Ca²⁺ begins to be restored to resting levels and this is followed by the generation of Ca²⁺ oscillations (images E through H). Cell boundaries are shown in white. The Ca²⁺response was measured from the large white square. During these Ca²⁺ responses the ciliary beat frequency (Yellow traces) was measured (from the small white square, right) for correlation with the Ca²⁺ changes. The waveform of the ciliary signal and Ca²⁺ changes during the initial spike (I) and a subsequent Ca²⁺ oscillation (J) are shown right. The lettered lines indicate the times at which the images (A - G) were acquired. During the initial Ca²⁺ increase (I and below), the ciliary beat frequency rapidly increased and reached a steady rate even through Ca²⁺ continued to increase.

It is important to note that only a small decrease in the ciliary beat frequency occurred even though the [Ca²⁺]_i had almost returned to a basal level. Furthermore, the subsequent Ca²⁺ oscillations were able to elevate the beat frequency to rates higher than that generate by the initial Ca²⁺ rises even though the associated Ca²⁺ increases were considerably less than the initial Ca²⁺ increase.


Hysteresis: Ca²⁺ verses ciliary beat frequency By separately analyzing increases and decreases in ciliary beat frequency with respect to Ca²⁺it became evident that changes in beat frequency were not a linear function of [Ca²⁺]_i. Although the increases in Ca²⁺ and beat frequency were well coupled, decreases in beat frequency often lagged behind or showed hysteresis with respect to decreases in [Ca²⁺]_i. During the initial Ca²⁺ increase, ciliary beat frequency rises to a maximum (red dots, arrow, top right) but during the decrease in Ca²⁺ the beat frequency remains constant ( white dots, arrow). Data pooled from multiple sequential Ca²⁺ oscillations (bottom right) indicates that the decrease in beat frequency does not occurs until a lower [Ca²⁺]_i is reached (white dots).

The relationship between Ca²⁺ (red trace) and beat frequency (yellow trace) is more clearly seen by analyzing a single Ca²⁺ oscillation (top, right). As Ca²⁺ increases (red dots, bottom), beat frequency increases. However, as the Ca²⁺ decreases (white dots), the decrease in beat frequency lags behind or shows hysteresis (bottom).