A soluble factor (less than 4000 Da) from chick spinal cord blocks slow hyperpolarizing afterpotentials in cultured rat muscle cells.

The slow hyperpolarizing afterpotential (slow HAP) following an evoked action potential in cultured rat muscle cells is inhibited by coculture with spinal cord neurons, spinal cord-conditioned medium (CM) and by a low-molecular-weight fraction (less than 4000 Da) prepared from this CM. The incidence, amplitude and duration of the slow HAPs decreased significantly after 24 h of incubation. In contrast, chick retinal neuron cocultures or CM did not alter the slow HAP.