Cardiac cells control transmitter release and calcium homeostasis in sympathetic neurons cultured from embryonic chick.

1. The contribution of target cells in controlling the functional properties of sympathetic neurons was investigated using pure neuronal cultures and co-cultures of neurons with their physiological target cells. 2. Chick embryo sympathetic neurons cultured alone exhibited maximal elevation of cytosolic free Ca2+ ([Ca2+]i) and release of tritiated noradrenaline ([3H]NA) when given ten stimulating pulses at 1 Hz but not at 10 Hz, yielding a negative frequency-release response. Stimulation-evoked release was only slightly enhanced by the K+ channel blocker tetraethylammonium (TEA, 10 mM). 3. When sympathetic neurons were co-cultured with cardiac cells of the chick embryo, electrically stimulated transmitter release and neuronal [Ca2+]i were reduced by 3- to 5-fold. Co-cultured neurons had a positive stimulation frequency--[3H]NA release response and 5- to 7-fold facilitation of release by TEA. 4. Voltage-clamped Ca2+ current density was decreased from 0.61 +/- 0.13 pA micron-2 in neurons alone to 0.19 +/- 0.03 pA micron-2 in co-cultured neurons. 5. Neonatal rat superior cervical ganglion (SCG) neurons were also relatively insensitive to TEA when cultured alone, but [3H]NA release was greatly facilitated by TEA when tested in SCG neurons co-cultured with rat neonatal cardiac myocytes. 6. The cardiac cell-induced changes in Ca2+ handling and release properties were produced within 24 h by sympathetic neuroeffector cells, but not by skeletal muscle cells or sensory neurons, and did not occur spontaneously in neurons grown alone for up to 6 days. 7. The frequency and TEA responses of neurons grown with cardiac cells are characteristic of responses seen in sympathetic neuroeffector organs. We conclude that physiological targets play a crucial role in development of normal transmitter-release properties by controlling Ca2+ homeostasis in sympathetic neurons.