Hubbard P C, Barata E N, Canario A V
Centro de Ciências do Mar, Universidade do Algarve, Campus de Gambelas, Portugal and Departamento de Biologia, Universidade de Evora, Apartado 94, Portugal.
J Exp Biol. 2000 Dec;203(Pt 24):3821-9. doi: 10.1242/jeb.203.24.3821.
Estuarine and/or migratory teleosts may experience large and rapid changes in external [Ca(2+)]. Previous studies have largely centred on the physiological mechanisms that maintain a constant plasma [Ca(2+)] in the face of such external fluctuations, but little work has been directed to examining how these changes may originally be detected. We present evidence that the olfactory system of the gilthead seabream (Sparus aurata) is highly sensitive to reductions in environmental [Ca(2+)] and suggest a possible mechanism by which this may be mediated. Multi-unit extracellular recordings were made from the olfactory nerve of Sparus aurata while the [Ca(2+)] of artificial sea water flowing over the olfactory epithelium was varied from 10 to 0 mmol l(-)(1). Reductions in [Ca(2+)] caused a large, non-accommodating increase in the firing rate of the olfactory nerve (apparent IC(50)=1.67+/-0.26 mmol l(-)(1), apparent Hill coefficient=-1.22+/-0.14; means +/- s.e.m., N=6). This response was not due to the concomitant reduction in osmolality and was specific for Ca(2+). During continuous exposure of the olfactory epithelium to Ca(2+)-free sea water, the apparent IC(50) and Hill coefficient in response to increases in [Ca(2+)] were 0.48+/-0.14 mmol l(-)(1) and -0.76+/-0.16 (means +/- s.e.m., N=6), respectively, suggesting an adaptation of the Ca(2+)-sensing system to low-[Ca(2+)] environments. Ca(2+) is intimately involved in signal transduction in the olfactory receptor neurones, but our data support a true olfactory response, rather than a non-specific effect to lowering of external [Ca(2+)]. The absence of Ca(2+) from sea water only partially and temporarily blunted the olfactory response to the odorant l-serine; the response amplitude recovered to control levels within 20 min. This suggests that the olfactory system in general is able to adapt to low-[Ca(2+)] environments. We suggest that the Ca(2+ )sensitivity is mediated by an extracellular Ca(2+)-sensing receptor similar to the recently characterized mammalian Ca(2+)-sensing receptor.
河口和/或洄游硬骨鱼可能会经历外部[Ca(2+)]的大幅快速变化。以往的研究主要集中在面对此类外部波动时维持血浆[Ca(2+)]恒定的生理机制上，但很少有研究致力于探究这些变化最初是如何被检测到的。我们提供证据表明，金头鲷（Sparus aurata）的嗅觉系统对环境[Ca(2+)]的降低高度敏感，并提出了一种可能的介导机制。在流经嗅觉上皮的人工海水的[Ca(2+)]从10 mmol l(-)(1)变化到0 mmol l(-)(1)时，对金头鲷的嗅神经进行多单位细胞外记录。[Ca(2+)]的降低导致嗅神经放电频率大幅、非适应性增加（表观IC(50)=1.67±0.26 mmol l(-)(1)，表观希尔系数=-1.22±0.14；平均值±标准误，N=6）。这种反应不是由于渗透压的同时降低，且对Ca(2+)具有特异性。在嗅觉上皮持续暴露于无Ca(2+)海水期间，对[Ca(2+)]增加的反应的表观IC(50)和希尔系数分别为0.4８±0.14 mmol l(-)(1)和-0.76±0.16（平均值±标准误，N=6），表明Ca(2+)传感系统适应了低[Ca(2+)]环境。Ca(2+)密切参与嗅觉受体神经元的信号转导，但我们的数据支持一种真正的嗅觉反应，而不是对外部[Ca(2+)]降低的非特异性效应。海水中缺乏Ca(2+)仅部分且暂时减弱了对气味剂L-丝氨酸的嗅觉反应；反应幅度在20分钟内恢复到对照水平。这表明一般的嗅觉系统能够适应低[Ca(2+)]环境。我们认为Ca(2+)敏感性是由一种细胞外Ca(2+)传感受体介导的，类似于最近鉴定的哺乳动物Ca(2+)传感受体。
