Evidence for an Oscillator Other than Luteinizing Hormone Controlling the Secretion of Progesterone in Cattle

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Abstract
The frequencies of pulses in ovarian and systemic concentrations of progesterone, systemic concentrations of luteinizing hormone (LH) and rate of ovarian blood flow were quantified and compared. Blood was collected simultaneously from previously implanted catheters in the ovarian venous circulation and jugular vein on Day 12 or 13 of estrous cycles from 4 nonlactating dairy cows. Blood was collected at a rate of 2.5 ml/min for 5 min out of every 10 min over an 8 h period. The mean rate of blood flow in the ovarian artery during the 5 min collection period was estimated by an electromagnetic blood flow transducer. Pulses were observed over time in both ovarian and systemic concentrations of progesterone at frequencies that ranged between 0.625 and 0.875 cycles/h (1.1-1.5 h/cycle) among the animals. Only 1 or 2 episodes of LH release were observed during the 8 h period, and transient increases in blood flow to the ovaries were associated temporally with each episode of LH release. The estimated frequencies for release of LH and increased blood flow were the same for each animal and ranged between 0.250 and 0.375 cycles/h. A 2nd cycle with a frequency similar to that for LH was evident in the spectral density functions for ovarian and systemic concentrations of progesterone. This cycle was eliminated when the cycle for LH was removed from the data for progesterone, but the magnitude and frequency of the pulses in progesterone were not affected. An oscillator other than LH may be partially responsible for regulation of the secretion of progesterone.