Fig. 4.

Leptin inhibits Ca²⁺ influx induced by 16.7 mM glucose in human β-cells. Two fura-2-loaded human β-cells were initially bathed in standard extracellular saline (SES containing 16.7 mM glucose) and exhibited irregular spikes of [Ca²⁺]_i resulting from influx during Ca²⁺-dependent action potentials. A stable baseline of [Ca²⁺]_i in 16.6 mM glucose was established visually and recorded for 5 min before lowering glucose to 5.6 mM and subsequent application of leptin. The bathing solution was then exchanged for 5.6 mM glucose SES, and this led to the inhibition of spiking activity, consistent with hyperpolarization of the β-cell membrane as the glucose concentration was reduced. Reintroduction of 16.7 mM glucose-SES plus 6.25 nM human leptin then led to an initial restoration of spiking activity that was immediately inhibited in the presence of leptin at this high glucose concentration. The upper and lower panels show two representative β-cell responses to leptin of eight cells so studied.