Patch-clamp-induced perturbations of [Ca(2+)](i) activity in somatotropes

Somatotropes and GC cells, a GH-producing cell line, exhibit [Ca(2+)](i) oscillations that result from rhythmic Ca(2+) action potentials. Determination of this operating mode required simultaneous recording of both parameters by fura-2 imaging and patch-clamp techniques. In order to test whether patch recording induces artificial alteration of the [Ca(2+)](i) oscillatory pattern, we recorded separately or simultaneously [Ca(2+)](i) and membrane potential. In the absence of any other stimulation, seal formation in patch-clamp recording evoked by itself a 2.5- to 4-fold persistent increase in basal [Ca(2+)](i), speeded up their frequency (from 0.03-0.17 to 0.4 Hz) and changed their pattern to a tonic mode. Patch-induced [Ca(2+)](i) increase was reproduced by mechanical contact between the pipette and the membrane. It was reduced by nifedipine, a blocker of L-type Ca(2+) channels, as well as by removal of external Na(+). It was fully blocked by external Ca(2+) removal or gadolinium. All patch-clamp-induced perturbations were reversed by membrane hyperpolarization. We propose that patch-clamp recording evokes Ca(2+) entry through L-type Ca(2+) channels either directly, or indirectly via membrane depolarization. This shows that patch recordings in endocrine cells showing mechanosensitivity have to be interpreted with caution, and explains why long-lasting patch recordings are so difficult to obtain.